Match new spec for non-null attributes with default values

[filipncs]

The spec was recently changed to clarify what to do with non-null attributes with default values:
graphql/graphql-spec#418

I've added a PR to illustrate how we don't live up to the new spec:
#1045

Thoughts? I think it should be a fairly simple fix for this case, but maybe this would cause other problems?

Note that this was a specific issue I ran into with trying to use the new spec, there could easily be other issues that should be fixed.

[filipncs]

I added a commit to the PR that performs the necessary change. Doesn't seem to obviously break anything else.

[bbakerman]

Thanks for reporting. I will look at the PR and comment there.

For the record there was a LOT of angst about defaulting null values in the new spec. A lot!