docs($anchorScroll): minor copyedit.

Singular-tense verb. Definite article.

Closes #7319
fix($httpBackend): Add missing expectHEAD() method

This was documented but not implemented.

With accompanying unit test to ensure the $httpBackend.expect*() methods exist.

Closes #7320
docs(guide/providers): minor edits
docs($injector): fix typos
chore(doc-gen): update to dgeni 0.3.0
feat(NgMessages): introduce the NgMessages module and directives