util.jid: Fix special escaping of '\' per XEP-0106

From XEP-0106 §2. Requirements:
> in certain circumstances, the escaping character itself ("\") might
> also be escaped

Later in §4.2 Address Transformation Algorithm it is stated that the
backslash would only be escaped if it forms an escape sequence. Thus
'\foo' is unaltered but '\20' must be escaped into '\5c20'.

Thanks to lovetox and jonas’ for brining up.

util/jid.lua

@@ -22,7 +22,11 @@ local escapes = {
 	["@"] = "\\40"; ["\\"] = "\\5c";
 };
 local unescapes = {};
-for k,v in pairs(escapes) do unescapes[v] = k; end
+local backslash_escapes = {};
+for k,v in pairs(escapes) do
+	unescapes[v] = k;
+	backslash_escapes[v] = v:gsub("\\", escapes)
+end
 
 local _ENV = nil;
 -- luacheck: std none
@@ -107,7 +111,7 @@ local function resource(jid)
 	return (select(3, split(jid)));
 end
 
-local function escape(s) return s and (s:gsub(".", escapes)); end
+local function escape(s) return s and (s:gsub("\\%x%x", backslash_escapes):gsub("[\"&'/:<>@ ]", escapes)); end
 local function unescape(s) return s and (s:gsub("\\%x%x", unescapes)); end
 
 return {