Supply language_id values to Neovim LSP API

Change logic so ALE's LSP implementation and the Neovim LSP client
retrieve the language_id for language clients at roughly the same time
via the same means. This makes ALE inform the language server what the
language for the language is for clients.

autoload/ale/assert.vim

@@ -205,7 +205,10 @@ endfunction
 function! ale#assert#LSPLanguage(expected_language) abort
     let l:buffer = bufnr('')
     let l:linter = s:GetLinter()
-    let l:language = ale#linter#GetLanguage(l:buffer, l:linter)
+    let l:Language = l:linter.language
+    let l:language = type(l:Language) is v:t_func
+    \   ? l:Language(l:buffer)
+    \   : l:Language
 
     AssertEqual a:expected_language, l:language
 endfunction

autoload/ale/linter.vim

@@ -446,9 +446,3 @@ function! ale#linter#GetAddress(buffer, linter) abort
 
     return type(l:Address) is v:t_func ? l:Address(a:buffer) : l:Address
 endfunction
-
-function! ale#linter#GetLanguage(buffer, linter) abort
-    let l:Language = a:linter.language
-
-    return type(l:Language) is v:t_func ? l:Language(a:buffer) : l:Language
-endfunction

autoload/ale/lsp.vim

@@ -5,9 +5,10 @@
 let s:connections = get(s:, 'connections', {})
 let g:ale_lsp_next_message_id = 1
 
-" Given an id, which can be an executable or address, and a project path,
+" Given an id, which can be an executable or address, a project path,
+" and a language string or (bufnr) -> string function
 " create a new connection if needed. Return a unique ID for the connection.
-function! ale#lsp#Register(executable_or_address, project, init_options) abort
+function! ale#lsp#Register(executable_or_address, project, language, init_options) abort
     let l:conn_id = a:executable_or_address . ':' . a:project
 
     if !has_key(s:connections, l:conn_id)
@@ -28,6 +29,7 @@ function! ale#lsp#Register(executable_or_address, project, init_options) abort
         \   'is_tsserver': 0,
         \   'data': '',
         \   'root': a:project,
+        \   'language': a:language,
         \   'open_documents': {},
         \   'initialized': 0,
         \   'init_request_id': 0,
@@ -677,9 +679,20 @@ function! ale#lsp#Send(conn_id, message) abort
     return l:id == 0 ? -1 : l:id
 endfunction
 
+function! ale#lsp#GetLanguage(conn_id, buffer) abort
+    let l:conn = get(s:connections, a:conn_id, {})
+    let l:Language = get(l:conn, 'language')
+
+    if empty(l:Language)
+        return getbufvar(a:buffer, '&filetype')
+    endif
+
+    return type(l:Language) is v:t_func ? l:Language(a:buffer) : l:Language
+endfunction
+
 " Notify LSP servers or tsserver if a document is opened, if needed.
 " If a document is opened, 1 will be returned, otherwise 0 will be returned.
-function! ale#lsp#OpenDocument(conn_id, buffer, language_id) abort
+function! ale#lsp#OpenDocument(conn_id, buffer) abort
     let l:conn = get(s:connections, a:conn_id, {})
     let l:opened = 0
 
@@ -693,7 +706,8 @@ function! ale#lsp#OpenDocument(conn_id, buffer, language_id) abort
             \    'client_id': l:conn.client_id,
             \})
         else
-            let l:message = ale#lsp#message#DidOpen(a:buffer, a:language_id)
+            let l:language_id = ale#lsp#GetLanguage(a:conn_id, a:buffer)
+            let l:message = ale#lsp#message#DidOpen(a:buffer, l:language_id)
             call ale#lsp#Send(a:conn_id, l:message)
         endif
 

autoload/ale/lsp_linter.vim

@@ -306,11 +306,10 @@ function! ale#lsp_linter#OnInit(linter, details, Callback) abort
     let l:command = a:details.command
 
     let l:config = ale#lsp_linter#GetConfig(l:buffer, a:linter)
-    let l:language_id = ale#linter#GetLanguage(l:buffer, a:linter)
 
     call ale#lsp#UpdateConfig(l:conn_id, l:buffer, l:config)
 
-    if ale#lsp#OpenDocument(l:conn_id, l:buffer, l:language_id)
+    if ale#lsp#OpenDocument(l:conn_id, l:buffer)
         if g:ale_history_enabled && !empty(l:command)
             call ale#history#Add(l:buffer, 'started', l:conn_id, l:command)
         endif
@@ -357,11 +356,21 @@ function! s:StartLSP(options, address, executable, command) abort
     let l:init_options = ale#lsp_linter#GetOptions(l:buffer, l:linter)
 
     if l:linter.lsp is# 'socket'
-        let l:conn_id = ale#lsp#Register(a:address, l:root, l:init_options)
+        let l:conn_id = ale#lsp#Register(
+        \   a:address,
+        \   l:root,
+        \   l:linter.language,
+        \   l:init_options
+        \)
         let l:ready = ale#lsp#ConnectToAddress(l:conn_id, a:address)
         let l:command = ''
     else
-        let l:conn_id = ale#lsp#Register(a:executable, l:root, l:init_options)
+        let l:conn_id = ale#lsp#Register(
+        \   a:executable,
+        \   l:root,
+        \   l:linter.language,
+        \   l:init_options
+        \)
 
         " tsserver behaves differently, so tell the LSP API that it is tsserver.
         if l:linter.lsp is# 'tsserver'