Use parse instead of format when calculating checked

[jharding]

This may be an incorrect assumption, but it seems like the getter for checked should be using parse, not format when comparing values. In addition to fixing an issue I was experiencing, the TypeScript definitions suggest this should be the intended behavior as well as parse returns FieldValue and state.config is of the type FieldValue.

I added a couple of tests to prevent future regressions – please let me know if there's anything else I should do to get this to a state where you'd be comfortable accepting the patch.

Thanks for the great library!

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Sandbox	Source
React Final Form - Simple Example	Configuration
React Final Form - Synchronous Record Level Validation	Configuration
React Final Form - Synchronous Field Level Validation	Configuration
final-form/react-final-form	Configuration
React Final Form - Subscriptions Example	Configuration

[nkappler]

value should probably be const as you can call parse right here, then you could remove line 164 and change 171 to `return value === _value"

[nkappler]

I'm not a maintainer, just interested in this PR as I stumbled upon it due to this bug #560