Add support for binary serializer storage

[sigmavirus24]

Some serializers return bytes instead of text and we need a way to indicate that when opening the cassette file.

Related-to gh-123

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cc @wimglenn for review

[hroncok]

The code looks reasonable. I would appreciate some kind of test tough.

[sigmavirus24]

Thanks for the prompt @hroncok. I added them in the latest commit.

[wimglenn]

This "corrected file mode" method seems to be a needless complexity, I prefer my implementation where it's just using plain old class attributes on the serializer. However, it's the same basic functionality provided, so LGTM

[sigmavirus24]

I prefer my implementation where it's just using plain old class attributes on the serializer.

There's something to be said for keeping things simpler for everyone. Your implementation is ideal if there's ever a case where a serializer wants to read text and write bytes or vice-versa, but that seems to be entirely unlikely. Simplifying what a serializer author needs to care about makes things harder to do wrong.