cover art archive now returns content type application/json

[alastair]

http://tickets.musicbrainz.org/browse/CAA-75

We have a hack in the caa code: https://github.com/alastair/python-musicbrainzngs/blob/master/musicbrainzngs/caa.py#L72

[mineo]

This has been (sadly) at the bottom of my todo list since I closed CAA-75. To implement a content type check we need to change _safe_read to return the file-like object returned by opener.open so we can access the headers. Unfortunately the docs don't really tell whether calling read can raise exceptions or if only open can - if read can raise some, moving that out of _safe_read would kind of defeat its purpose :)

[sampsyo]

Sadly, I believe read can raise errors (I'm about 75% confident about that).

See also #123. 😃