Allow to mix JSON_STRICT with optional keys

On unpack, one may want to mix `JSON_STRICT` and optional keys by using
a format like `{s:i,s?o!}`. Unfortunately, this fails the stric test
with `-1 object item(s) left unpacked` error when the second key is not
specified.

To fix that, we iter on each key and we check if we have successfully
unpacked them. This is less efficient than the previous method but it
brings correctness.
Micro-optimization for JSON_STRICT when no optional key is used

The previous commit introduced a loop on all input keys to check the
strict mode. We can avoid this if we don't expect an optional key. In
this case, we fallback to the previous method to compare the length of
the set of expected keys and the length of the parsed keys.
Merge pull request #163 from vincentbernat/fix/unpack-mix-optional-and-strict

Fix unpack when mixing strict mode and optional keys