Add onSend callback support for fixtures

[calderas]

This is helpful to inspect xhr requests when doing integration tests but you don't have access to the actual request object directly.

I've done something similar with sinon.js using filters and inspecting the onCreate/onSend callbacks but this is a much cleaner implementation.

JS Bin: http://emberjs.jsbin.com/ricome/3/edit

[alexlafroscia]

Is there a reason that this wasn't added in? It would be really useful for integration testing!

[CLAassistant]

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