Adds support for invoking the on_error handler.

[hsume2]

Hi,

When writing tests for an EventMachine client, I wanted to ensure that on_error handlers were properly triggered.

This PR adds support for to_errback:

stub_request(:get, "www.example.com").to_errback

Which will then invoke the on_error handler.

[grosser]

👍

[bblimke]

Thank you for this pull request and suggesting this feature.
WebMock purpose is to provide http client independent mocking API. This change adds EM only method to WebMock generic API, which unfortunately makes it useless for all other http clients except EM.
WebMock API has to_raise method. Maybe it's possible to implement to_raise behaviour in EM adapter instead?

[hsume2]

@bblimke thanks for the suggestion, that makes sense. I'll give that a shot and see if I can get anywhere with that :)

[bblimke]

to_errback is only em-http-request specific so this pull request can't be merged in this form.