Verify that literal values are valid when decoding

[roshcagra]

I have some fields in my dataclasses that are large unions, so I've tried setting single-value literals on them to help the decoder discriminate between them, but it looks like while literals are supported, the values aren't validated during decoding.

For example, if you have two dataclasses:

@dataclass
class Foo(JsonSchemaMixin):
     common_field: int
     name: Literal['Foo'] = 'Foo'

@dataclass
class Bar(JsonSchemaMixin):
     common_field: int
     name: Literal['Bar'] = 'Bar'
     other_field: Optional[int] = None

and a dataclass that uses them as a union type,

@dataclass
class Baz(JsonSchemaMixin):
     my_foo_bar: Union[Bar, Foo]

Then

Baz.from_dict(
   Baz(my_foo_bar=
      Foo(common_field=1)
   ).to_dict()
)

will result in

Baz(my_foor_bar=Bar(common_field=1))

even though the literals should help disciminate.

[roshcagra]

I've got a fix for this! Will submit the PR ASAP

[s-knibbs]

This is pretty closely related with #170