move _encode_binary to jsonutil.encode_images

[minrk]

And fix identification of PNG/JPEG data, which could be double-encoded when using a JSON library that decodes to str instead of unicode (simplejson 2.6, for instance).

The function is also moved to jsonutil.

Changes are tested.

This is critical for 0.13, because the parallel magics cannot republish images without it, depending on whether simplejson is present.

[minrk]

More specifics:

_encode_binary checked if the data was bytes (str on Python 2). However, this data could still be base64-encoded str if simplejson is in use, rather than unicode, in which case the republishing would double-encode the data. This, instead, actually checks the first few bytes to see if these are in fact jpeg/png data.

Some alternatives:

1. handle this in json_clean, so that images anywhere in any message will be b64-encoded
2. reverse the test, so that rather than checking the first few bytes of data against proper PNG/JPG, check them against b64-encoded PNG/JPEG. The main difference being how garbage data would be handled.

[fperez]

Test results for commit 0014248 merged into master
Platform: linux2

• python2.7: OK
• python3.2: OK (libraries not available: cython matplotlib oct2py pygments pymongo rpy2 wx wx.aui)

Not available for testing: python2.6, python3.1

[fperez]

Since this is now public, the docstring should explain what format_dict is meant to contain.

[fperez]

Tiny docstring fix, otherwise looks good. thanks!

[minrk]

Thanks - docstring updated, merging.