Add MessageExpectation cop

[backus]

Enforces preferred styles of either

expect(foo).to receive(:bar)

or

allow(foo).to receive(:bar)

Fixes #23

[andyw8]

I'm not sure about this practicality of this one. Won't the vast majority of specs in any project need to use expect?

See https://relishapp.com/rspec/rspec-mocks/docs/basics/spies for some examples of the situations I had in mind for #23.

[backus]

@andyw8 I've run this on two projects now and it actually seems very effective. Basically this should be one piece of the puzzle in enforcing one of these two styles:

allow(foo).to receive(:bar)

# exercise system under test

expect(foo).to have_received(:bar)

expect(foo).to receive(:bar)

# exercise system under test

Note that this cop will never flag have_received. It simply says either "add spies before exercising your code" or "add a message expectation before exercising your code."

The second piece of the puzzle (which I haven't implemented yet) would be to find tests where spies are setup but not verified within that test. Basically it could detect (and autocorrect) the following:

allow(foo).to receive(:bar)
^^^^^^^^^^ Message expectation specified but not verified

thing.do_something(foo)

which could be autocorrected to

allow(foo).to receive(:bar)

thing.do_something(foo)

expect(foo).to have_received(:bar)