Fix garbage collection for Shotgun instances

[bchapman]

Found that Shotgun instances weren't getting garbage collected.
Did some research and found that it's due to the this line in int

if ensure_ascii:
    self._json_loads = self._json_loads_ascii

This causes python not to garbage collect any Shotgun instances

Example:

>>> import gc
>>> sg = shotgun_api3.Shotgun(**params)
>>> gc.get_referrers(sg)
[<bound method Shotgun._json_loads_ascii of <trak.shotgun.Shotgun object at 0x10542dd10>>, ...]

Changed it to store the ensure_ascii as variable on the class and check in _json_loads which method to use.

[kporangehat]

Hey Brennan,

Thanks for the insight and the PR! We were mostly offline last week at our yearly summit so apologies for not chiming in sooner. This change does look pretty innocuous. We'll assign someone to run the tests against it and try and bring it in. Will follow up shortly.