feat(mocks): change MockHttpBackend to define assertions on flush.

[vsavkin]

Add new methods (flushGET, flushPOST, etc) to the MockHttpBackend class to allow defining assertions on flush. These methods add a new expectation and then flush all the pending requests until they find the one matching the expectation.

BREAKING CHANGE:

Unexpected requests are detected only when flush is called.

Before:

    backend("GET", /some"); //throws here if `/some` has not been defined

After:

    backend("GET", /some"); //no problem, just records the request
    backend.flush(); //throws here

Closes #900, #1417

[vsavkin]

Some notes:

I've added flushGET, flushPOST, etc.

These new functions add a new expectation and then flush all the pending requests until they find the one matching the expectation.

    hb.when("GET", '/first').respond("OK");

    hb("GET", '/first', callback);
    hb("GET", '/second', callback);
    hb("GET", '/third', callback);

    hb.flushGET("GET", "/second").respond("OK"); // flushes "/first" and "/second"
    hb.flushGET("GET", "/third").respond("OK"); // flushes "/third"

I haven't deleted the expect function, so you can still write:

    hb.expect("GET", '/first').respond("OK");
    hb("GET", '/first', callback);
    hb.flush();

I can delete expect if you think it is not needed any more.

[jbdeboer]

Awesome; could you add a second commit to the PR converting expects to flushGETs in the tests?

[jbdeboer]

This is a breaking change, so we need an upgrade plan.

Setting this to "reviewing" until then.

[vsavkin]

@jbdeboer This will cause issues only if there are tests that depend on the fact that the exception won't be thrown until flush. I can run the tests to see if it causes issues. If it does, we can easily change the tests to call flush.

[jbdeboer]

@vsavkin is reviewing this change and how it effects apps.