Fix Collections stuck as partial object after reloading

[JonnyWong16]

Description

Fix Collections stuck as partial object after reloading.

• Removed isFullObject() from Playable.
• Check if either _details_key or key is equal to the _initpath.

From my testing, I don't think this breaks any existing functionality. Should there be tests added to check isFullObject() for all the other media types similar to movies (https://github.com/pkkid/python-plexapi/blob/master/tests/test_video.py#L66)?

Fixes #605

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable