feat(ngMock/$httpBackend): support a matching function for data param

Add support for passing function as validating data:
 - To avoid hacking test method of RegExp
 - Optionally overwrite `toString` method of fn to show validation tips
 - change docs: param description for `when`, `whenPost`, `whenPut`,
   `expect`, `expectPost`, `expectPut`, `expectPATCH`

Closes: #2981