tests: rename variables for consistency

expect/actual is a common idiom.
"pickle" reads better than "pickled".

Signed-off-by: David Aguilar <davvid@gmail.com>
tests: add coverage for using None as a dict key

Related-to: #91

Signed-off-by: David Aguilar <davvid@gmail.com>
tests: rename s/tests/test/ for consistency

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

Signed-off-by: David Aguilar <davvid@gmail.com>
jsonpickle: add convenient dumps() and loads() aliases

Signed-off-by: David Aguilar <davvid@gmail.com>
backend: return a success bool from load_backend()

Return True when load_backend() succeeds.  Simplify JSONBackend by
moving common arguments into load_standard_backend().

Signed-off-by: David Aguilar <davvid@gmail.com>
pickler: escape string that have special meaning when used as dictionary key
refactor check order in _flatten_key_value_pair