Add check that literal values are valid to decode_field

[roshcagra]

Adds an explicit check that a literal field has a valid value when decoding. This will improve decoding when types in a Union have overlapping fields but may be discriminated by a literal.

[roshcagra]

Fix for https://github.com/s-knibbs/dataclasses-jsonschema/issues/163

[roshcagra]

@s-knibbs Looks like I can't add you as an assignee, but hope this looks good!

[s-knibbs]

Fixes #163

[roshcagra]

@s-knibbs updated re: your feedback