Avoid NameError when types are nested
Use String#constantize instead of Kernel#const_get

String#constantize offered by activesupport provides reliable constant
name resolution for most versions of MRI. Tested on 1.9.3 and 2.1.2.
Add spec for case where type name resolution fails
Merge pull request #66 from inbeom/name-collision

Preventing NameError when types are nested