Allow default behavior with HTTP interceptors

[Drynwynn]

In our project we wanted to be able to POST to a certain path we had mocked using the in-memory-web-api, but wanted to retain the ability for POST to add collection items without having to reinvent the wheel. It was not immediately apparent if our service had access to its controlling service object to call the collectionHandler() method, so we just modified the service to accept a null return to continue default processing. Submitting here in case it is found useful to others.

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[Drynwynn]

I signed it!

…

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[googlebot]

CLAs look good, thanks!

[wardbell]

I really like your idea.

I'm refactoring heavily to support Angular's "new" HttpClient so I can't merge your code exactly as written. I'll have to close this PR.

But I'm definitely taking it as you can see in PR #130. Thanks!

[wardbell]

Closing for reason give in my review. See PR #130

[Drynwynn]

Thanks Ward! Good luck on the refactor.