Bugfix: Correctly identify the type of content update notification to use for each LSP server

[rgoya]

Fixes #1547 <- go here for a full description of the bug.

The s:file_content object has an entry for each buffer, and each buffer entry has an entry for each LSP server it has been synced with:

vim-lsp/autoload/lsp.vim

Lines 9 to 20 in f7ccf00

	" This hold previous content for each language servers to make
	" DidChangeTextDocumentParams. The key is buffer numbers:
	" {
	" 1: {
	" "golsp": [ "first-line", "next-line", ... ],
	" "bingo": [ "first-line", "next-line", ... ]
	" },
	" 2: {
	" "pylsp": [ "first-line", "next-line", ... ]
	" }
	" }
	let s:file_content = {}

When deciding whether an LSP has been sent a copy of the buffer or not, the current code only inspects whether the buffer entry exists. It should, instead, additionally verify whether the entry for the corresponding LSP server exists.

This PR adds such a check.