Correct out-of-spec Avro schemas with typing.Union

[xgamer4]

This pull request corrects #85 by checking the type of the provided default and placing it at the front of the type field in Avro schema. It adds two tests - one to reproduce the problem detailed in Issue 85 to develop against, and one to add a test to explicitly document the behavior of a typing.Optional[T] field without a default. The behavior of a typing.Optional[T] field remains unchanged. The existing test for union fields with defaults was updated.

Documentation still needs to be updated, and notes on the change (it's likely a breaking change, and notes on the severity) need to be added.

I wouldn't be surprised if there were minor merge conflicts with #86 as well - the two branches were developed independently of each other - so I'll need to reconcile those if/when one of these two merge requests gets approved.

[xgamer4]

Testing against the released v0.19.0 for breaking changes, I built a class like this:

@dataclass
class RandomSentence(faust.Record, AvroModel, serializer='avro_randomsentence'):
    """Randomly Generated Sentence. Purely for testing during initial dev"""
    sentence: str
    generated: datetime
    test: typing.Optional[str] = None
    test2: typing.Union[str, int] = 0

Ran code against it to have the schema version saved to the schema registry (under full compatibility). I then altered the model to this, to emulate the changes made under this pull request:

@dataclass
class RandomSentence(faust.Record, AvroModel, serializer='avro_randomsentence'):
    """Randomly Generated Sentence. Purely for testing during initial dev"""
    sentence: str
    generated: datetime
    test: typing.Union[None, str] = None
    test2: typing.Union[int, str] = 0

And the change went through and was saved to the schema registry as a new version, as appropriate.

[codecov-io]

Codecov Report

Merging #87 (e4f55cb) into master (edfa15d) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   99.55%   99.85%   +0.29%     
==========================================
  Files           7        7              
  Lines         671      678       +7     
  Branches      106      108       +2     
==========================================
+ Hits          668      677       +9     
+ Misses          1        0       -1     
+ Partials        2        1       -1     

Impacted Files	Coverage Δ
dataclasses_avroschema/fields.py	100.00% <100.00%> (+0.49%)	⬆️

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[marcosschroh]

@xgamer4 amazing! Nice work 👍 . Do you want to update the documentation as well?

[xgamer4]

@marcosschroh Thanks! Documentation has been updated - I added in a bit of explanation to the complex types section on Unions and updated the example.

EDIT: After a bit of thought I added in a note on non-breaking changes that generate a new schema version below the note on breaking changes. In practice I'm not sure if those matter, but it seemed worth noting that a new version will wind up updating schema versions without any changes on the dev's side.