Allow Instance to initialize from dict with '__class__' specified

[fwkoch]

This is only valid if __class__ is identical to the instance_class name. If more complicated behavior is required, deserialize should be used.

As an example, say you have:

import properties

class Blah(properties.HasProperties):
    a = properties.Integer('')

class HasBlah(properties.HasProperties):
    b = properties.Instance('', Blah)

previously this would be fine:

val = HasBlah(b={'a': 5})

but this would fail:

val = HasBlah(b={'a': 5, '__class__': 'Blah'})

instead requiring:

val = HasBlah.deserialize({'b': {'a': 5, '__class__': 'Blah'}})

With this PR, all of the above are valid.

[codecov[bot]]

Codecov Report

Merging #255 into dev will decrease coverage by 0.03%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev     #255      +/-   ##
==========================================
- Coverage   96.24%   96.21%   -0.04%     
==========================================
  Files          16       16              
  Lines        2319     2322       +3     
==========================================
+ Hits         2232     2234       +2     
- Misses         87       88       +1

Impacted Files	Coverage Δ
properties/base/instance.py	95.6% <66.66%> (-0.99%)	:arrow_down:

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[fwkoch]

I'm rejecting - this conflates initialization and deserialization. On init, we shouldn't see __class__, if we have a dictionary with that present, we should be deserializing instead (since that key would be created with serialize).