Validate input object not being an Enum

[rewritten]

We have found out that in a query like this, when the complexArg argument is an input object:

{ someField(complexArg: SEEMS_AN_ENUM) }

the argument is discarded instead of being marked as an error like if it was a scalar.

This PR adds a parse check for Input.Enum for this case, and tests for this and other cases.

[benwilson512]

Oops, great catch!

[rewritten]

Thanks to @ssaenz and @JamieMaclean for spotting the issue.