feature: make a best guess at prettifying object that have no prototype

[variousauthors]

Hello!

I'm working with a third party library that initializes objects using Object.create(null) (graphql). The result is that these objects do not have prototypes and do not implement toString. This becomes an issue when trying to validate object using your (otherwise wonderful) library.

I have added a line that checks whether a given object implements toString and tries to fall back on JSON.stringify. If stringify encounters the dreaded cyclic structure, then at that point I would say maybe the user needs to rethink things.

I've added two tests to capture this new behaviour. I think that this is not a breaking change. Please let me know if you have any questions or feedback!

For some background on this issue, see discussion here: graphql/express-graphql#177

[coveralls]

Coverage remained the same at 100.0% when pulling 9ae2a1b on variousauthors:master into e913a41 on ansman:master.