Make FetchRequest not try and parse a 204 response body

It currently checks for the presence of a Content-Type response header
and if present assumes that trying to parse the response will succeed.

Realtime currently sends a 204 (No Content) with a Content-Type header
from the POST rest.ably.io/push/publish endpoint when using HTTP 1.1
(see REA-1924).

(This bug hasn't been caught by the tests because I guess most of the
time they use HTTP 2 in browsers, and also we don't use FetchRequest in
the tests except briefly in those for the modular variant of the
library. I’ve created #1772 for improving this situation).

The 204 handling approach is copied from that which we already have in
XHRRequest.

Resolves #1771.

src/platform/web/lib/http/request/fetchrequest.ts

@@ -69,6 +69,10 @@ export default async function fetchRequest(
 
       clearTimeout(timeout!);
 
+      if (res.status == 204) {
+        return { error: null, statusCode: res.status };
+      }
+
       const contentType = res.headers.get('Content-Type');
       let body;
       if (contentType && contentType.indexOf('application/x-msgpack') > -1) {