Relax the requirement for a content-type on .well-known (#1824)

The main reason for this is that Apache etc won't stick a content-type on by
default, because they don't know it's JSON, so requiring it seems like it will
require unnecessary hoop-humping for everyone who wants to use a .well-known.

proposals/1708-well-known-for-federation.md

@@ -43,9 +43,9 @@ certificate validation, and following 30x redirects (being careful to avoid
 redirect loops). If the request does not return a 200, continue to step 4,
 otherwise:
 
-The response must have a `Content-Type` of `application/json`, and must be
-valid JSON which follows the structure documented below. Otherwise, the
-request is aborted.
+The response must be valid JSON which follows the structure documented
+below. Otherwise, the request is aborted. It is NOT necessary for the response
+to have a `Content-Type` of `application/json`.
 
 If the response is valid, the `m.server` property is parsed as
 `<delegated_server_name>[:<delegated_port>]`, and processed as follows: