web_renderjs_ui: fix detection of Base_redirect redirections

The check was made on the blob response type, which is set from the
Content-Type header returned by the server, but Safari has a different
interpretation of the charset parameter from the mime type, with a
content type set to application/json;charset=utf-8 like Base_redirect
does today, safari creates a blob with type application/json;charset=utf-8
and this was not detected as redirection and the json returned by
Base_redirect was downloaded. Fix this by checking only the essence
of the type.

This also revealed a potential problem when actually downloading json
files, in that case we also check that we have the X-Location header,
that is supposed to be set by Base_redirect before interpreting the json
and when it's not present we force download.