Nested generic type issue for GuardClauseAssertion

This PR addresses the problem of nested generic type for `GuardClauseAssertion`.

By the way, the first commit of this PR rearranges white spaces, so if you want to review, please check out [only the second commit](https://github.com/jwChung/AutoFixture/commit/ec427fd1eddd079d31748eb33f8c639e0935345a).