fix($http): add missing shortcut methods and missing docs

There was some inconsistency in version 1.2.25 regarding the definition
and documentation of shortcut methods in `ng.$http` and
`ngMock[E2E].$httpBackend`. Some methods didn't exist (although documented
as existing), whereas some methods did exist, but wheren't documented.
This commit fixes the above errors and adds tests to verify the existence
of all shortcut methods.
More specificcally, the following issues were addressed:
`ng.$http`: Add the missing `patch()` shortcut method and related docs.
`ng.$http` spec: Add test to verify the existence and functionality of the
`patch()` shortcut method.
`ngMock[E2E].$httpBackend`: Add docs for the (existing) `whenPATCH()`
shortcut method, add the missing `expectHEAD()`/`whenHEAD()` shortcut
methods, fix grammatical errors.
`ngMock[E2E].$httpBackend` spec: Add test to verify the existence of shortcut
methods for all HTTP verbs.

Closes #9180