Add HTTP status name to output of tests

[cllns]

Earlier today I came across a test failure, that said something along the lines of:

Expected response to be a <success>, but was a <401>

I didn't recognize 401 as being Unauthorized at first, so I had to go look it up.

Rails, through Rack, knows what a 401 means. This patch leverages that. It turns the above output into something more helpful:

Expected response to be a <2XX: success>, but was a <401: Unauthorized>

At first I thought I'd only provide the name if the name was asserted (e.g. :not_found would show :not_authorized, but 404 would show 401) but I can't think of a case where it's not helpful to have both.

The code to implement simply that is done in the first commit. The remainder of the commits are refactorings which move the logic of moving between symbol and code into an AssertionResponse class, which also simplifies the logic of assert_response, which was relatively gnarly.

[rails-bot]

r? @eileencodes

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[cllns]

This line means that we're creating a new AssertionResponse for each time code_with_name is called. This happens twice in generate_response_message, once for the expected and once for the actual. It's not ideal to create another response object for the Actual, since we already have one, but if we didn't we'd have to add the code for converting between the response code and the response name into the actual Response object.

[cllns]

Just fixed the tests :) and rebased on master.

[rafaelfranca]

Instead of autoloading I think it is better to require the file where it is needed.

[rafaelfranca]

Ah, actually it is better to autoload. 👍

[rafaelfranca]

Could you squash your commits?

[cllns]

All set @rafaelfranca! Also added a CHANGELOG note.