pickler: escape string that have special meaning when used as dictionary key
refactor check order in _flatten_key_value_pair
handle {None: None} to pickle as {"null": null}
Merge pull request #91 from lieryan/master

pickler: escape string that have special meaning when used as dictionary key

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #93 from lieryan/fix_none_key

Add special handling for `None` dictionary keys

Signed-off-by: David Aguilar <davvid@gmail.com>
tests: avoid assertIn for python2.6 compatibility

Signed-off-by: David Aguilar <davvid@gmail.com>
pickler: special-case None dict keys for demjson

Add backend tests to ensure that the expected behavior has coverage
across all backends.

Signed-off-by: David Aguilar <davvid@gmail.com>