Fix sending to a stream with a space in its name.

Previously if you tried to send to "a b", we actually ended up trying
to send to "a%20b", since we were url-encoding the stream name and
then not properly decoding it.

(imported from commit 307d2999bd309e47fc654ae4422ab4372edde064)

zephyr/static/js/compose.js

@@ -102,7 +102,8 @@ function check_stream_for_send(stream_name) {
     var okay = true;
     $.ajax({
         type: "POST",
-        url: "/json/subscriptions/exists/" + stream_name,
+        url: "/json/subscriptions/exists",
+        data: {'stream': stream_name},
         async: false,
         success: function (data) {
             if (data.exists === "False") {