Add setInput function to CakeRequest

Modify data originally from `php://input`. Useful for altering json/xml
data in middleware or DispatcherFilters before it gets to
RequestHandlerComponent or other controllers.
Added unit test for CakeRequest::setInput

Don't use mocks as the tests weren't really testing anything when mocks
were involved.

Refs #3764
Merge branch 'request-set-input' into 2.6

Fixes #3764
Merge branch '2.6' of github.com:cakephp/cakephp into 2.6