Use a copy of the server list when restarting (#540)

If the original server list is used, it is cleared during the step of
removing filetype buffers, so no servers are actually restarted, as the
`lspservers` list is empty by the time the `for` loop is reached.

autoload/lsp/lsp.vim

@@ -547,7 +547,7 @@ enddef
 
 # Restart the LSP server for the current buffer
 def RestartServer()
-  var lspservers: list<dict<any>> = buf.CurbufGetServers()
+  var lspservers: list<dict<any>> = buf.CurbufGetServers()->copy()
   if lspservers->empty()
     util.WarnMsg($'No Lsp servers found for "{@%}"')
     return