nsqadmin: fix js source-map serving for firefox

The Content-Type of the source-map file is not supposed to matter.
But, if Content-Encoding is gzip, and Content-Type is x-gzip, then
Firefox does not decompress the response before trying to decode json,
and fails to load the source map for the debugger, complaining:
"unexpected character at line 1 column 1 of the JSON data".
With a proper Content-Type, Firefox decompresses and understands.
(Chrome works either way.)

nsqadmin/http.go

@@ -173,6 +173,8 @@ func (s *httpServer) staticAssetHandler(w http.ResponseWriter, req *http.Request
 	ct := mime.TypeByExtension(ext)
 	if ct == "" {
 		switch ext {
+		case ".map":
+			ct = "application/json"
 		case ".svg":
 			ct = "image/svg+xml"
 		case ".woff":