LSP configuration via didChangeConfiguration (#1852)

* adding LSP configuration via workspace/didChangeConfiguration

test/lsp/test_lsp_client_messages.vader

@@ -175,6 +175,22 @@ Execute(ale#lsp#message#Hover() should return correct messages):
   \ ],
   \ ale#lsp#message#Hover(bufnr(''), 12, 34)
 
+Execute(ale#lsp#message#DidChangeConfiguration() should return correct messages):
+  let g:ale_lsp_configuration = {
+  \ 'foo': 'bar'
+  \ }
+  AssertEqual
+  \ [
+  \   0,
+  \   'workspace/didChangeConfiguration',
+  \   {
+  \     'settings': {
+  \         'foo': 'bar',
+  \     }
+  \   }
+  \ ],
+  \ ale#lsp#message#DidChangeConfiguration(bufnr(''), g:ale_lsp_configuration)
+
 Execute(ale#lsp#tsserver_message#Open() should return correct messages):
   AssertEqual
   \ [