LibWeb: Implement XMLHttpRequest.overrideMimeType

This allows you to ignore the Content-Type returned by the server and
always parse the content as if it's the given MIME type.

This will currently be used for allowing you to override the charset
of text responses.

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl

@@ -18,6 +18,7 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
 
     ByteString? getResponseHeader(ByteString name);
     ByteString getAllResponseHeaders();
+    undefined overrideMimeType(DOMString mime);
 
     attribute EventHandler onreadystatechange;