Update api_utils.py

Fixes #25 and the problem seen below:

Exception ValueError with message "No JSON object could be decoded" is being raised when running the tests create_user_channel_open and create_user_channel_authorized against surevine.com.

The root of the problem is that queries for a node's metadata is returning status 200 even if that node doesn't exist.

It means it's deffo a bug somewhere else, as the specs (https://buddycloud.org/wiki/buddycloud_HTTP_API#.2F:channel.2Fmetadata.2F:node) say it should return status 404 in that case.

Now, to fix the issue, the tests impacted by this commit actually rely on checking if the response's content is actually in valid JSON format before considering the query for metadata a success. (Maybe it should also check for the expected keys within that JSON object).