Fix 3207 - do not send didSave notification if not supported (#3930)

test/lsp/test_did_save_event.vader

@@ -90,6 +90,30 @@ Given foobar (Some imaginary filetype):
 Execute(Server should be notified on save):
   call ale#events#SaveEvent(bufnr(''))
 
+  AssertEqual
+  \ [
+  \   [1, 'textDocument/didChange', {
+  \     'textDocument': {
+  \       'uri': ale#path#ToURI(expand('%:p')),
+  \       'version': g:ale_lsp_next_version_id - 1,
+  \     },
+  \     'contentChanges': [{'text': join(getline(1, '$'), "\n") . "\n"}],
+  \   }],
+  \ ],
+  \ g:message_list
+
+Execute(Server should be notified on save with didSave is supported by server):
+
+  " Replace has capability function to simulate didSave server capability
+  function! ale#lsp#HasCapability(conn_id, capability) abort
+      if a:capability == 'did_save'
+          return 1
+      endif
+      return 0
+  endfunction
+
+  call ale#events#SaveEvent(bufnr(''))
+
   AssertEqual
   \ [
   \   [1, 'textDocument/didChange', {