Make languageId configurable per lsp (#880)

* Make languageId configurable per lsp

* Fix linter warnings

* Use function to define languageId

autoload/lsp.vim

@@ -880,9 +880,12 @@ function! s:get_text_document_text(buf, server_name) abort
 endfunction
 
 function! s:get_text_document(buf, server_name, buffer_info) abort
+    let l:server = s:servers[a:server_name]
+    let l:server_info = l:server['server_info']
+    let l:language_id = has_key(l:server_info, 'languageId') ?  l:server_info['languageId'](l:server_info) : &filetype
     return {
         \ 'uri': lsp#utils#get_buffer_uri(a:buf),
-        \ 'languageId': &filetype,
+        \ 'languageId': l:language_id,
         \ 'version': a:buffer_info['version'],
         \ 'text': s:get_text_document_text(a:buf, a:server_name),
         \ }

doc/vim-lsp.txt

@@ -710,6 +710,7 @@ one parameter which is a vim |dict| and is refered to as |vim-lsp-server_info|
 		\ 'blocklist': ['filetype to blocklist'],
 		\ 'config': {},
 		\ 'workspace_config': {'param': {'enabled': v:true}},
+		\ 'languageId': {server_info->'python'},
 		\ })
 	endif
 <
@@ -730,6 +731,7 @@ The vim |dict| containing information about the server.
 	'blocklist': ['filetype'],
 	'config': {},
 	'workspace_config': {},
+	'languageId': {server_info->'filetype'},
     }
 <
  * name:
@@ -807,6 +809,16 @@ The vim |dict| containing information about the server.
     Example: >
 	'workspace_config': {'pyls': {'plugins': \
 		{'pydocstyle': {'enabled': v:true}}}}
+<
+  * languageId:
+    optional function returning |string|
+    By default the languageId is the current filetype. If you're using a sub
+    filetype like 'ios.swift' your language server may not return anything
+    because it does not know this language.
+    In this case you might want to overwrite the languageId with this key.
+
+    Example: >
+	'languageId': {server_info->'typescript'}
 <
   * config:
     optional vim |dict|