Fix invalid accept header in put saver (#8547)

IIUC the charset doesn't belong in the Accept header. It does belong
in a Content-Type header though, see [1] and [2].

FYI this header causes problems for Tiddlyhost which must work around
the invalid mime type, see [3] for the nitty-gritty.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept
[2] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type
[3] 10cd353552

core/modules/savers/put.js

@@ -20,7 +20,7 @@ Retrieve ETag if available
 */
 var retrieveETag = function(self) {
 	var headers = {
-		Accept: "*/*;charset=UTF-8"
+		Accept: "*/*"
 	};
 	$tw.utils.httpRequest({
 		url: self.uri(),