Fix (complement) for array reversed problem

The function to_value should be fixed, otherwise arrays in json would appear
inverted in erlang (the problem wasn't only related to proplists). Besides,
the eunit test showing this problem was missing, and the record associated
with it was using a N-tuple rather than a list. (N-tuple is an invalid
datatype for mochijson2). I fixed the record definition and implemented
the test.
Replace mochijson2 for jiffy and fix support for undefined record members.

Jiffy is a much better performing json parser and its convertion table
is very similar to mochijson2's.

Regarding the fix of undefined record members. The problem was that
undefined members were encoded with a string value of "undefined"
leading to a problem when decoding back. In order to fix this
situation, undefined record members are skipped during encoding.
This is compatible with the decoder behavior where a member would not be
assigned if it's missing in the json structure. A record member assigned to
the atom null is encoded properly and decoded back to the same atom null.
Elaborate the test a little more.