Convert StrongParameters cache to a hash. This fixes an unbounded memory leak

demonstrated on @tenderlove's latest blog post:

http://tenderlovemaking.com/2014/06/02/yagni-methods-are-killing-me.html
The test is still there, but now it doesn't have a shitty error message when it fails.

https://github.com/seattlerb/minitest/blob/master/lib/minitest/assertions.rb#L336