Argument with wrong value type do not raise an error

Currently, setting the value of an input with the wrong type is not raising an error.
For example if the input as the type `UUID` and that you provide the value `2` it doesn't fail.

There are various cases like that (see tests in the pull request for an extensive list).

This is due to the fact that now no value is represented by `Undefined` and not `None` in graphql-core.