Make assert_dom_equal ignore insignificant whitespace

[joshpencheon]

Following on from issue #62 and PR #66, I've taken a stab at implementing this. Not suggesting this ready to go, but just trying to keep the discussion alive.

In the past, we've had crude downstream implementations, but I've tried to get something that works with the current Rails approach, and behaves something like an inline formatting context.

Happy to keep working on this, or hand over to @chancancode or another.

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[joshpencheon]

I've changed the approach taken, to try and make behaviour comparable to @chancancode's suggestion on #62:

is it okay that it "mostly does what you would expect", but isn't exactly the same as what happens in the browser's rendering engine (and there will be a strict mode for the current behavior)

I haven't implemented the "strict mode", but have hooked in to the existing logic in such a way as to make it trivial if the API is agreed on, e.g:

assert_dom_equal(expected, actual, message=nil, strict: false)

The JRuby build is failing because of unrelated issues, possibly this: travis-ci/travis-ci/issues/8446

[jduff]

With #84 merged I think it covers the case this PR was for, please give master a try and see if there were any cases missed!

[joshpencheon]

Thanks @jduff ! I see test coverage overlaps, and I've tested master against our projects and it's working fine. :)

Closing in favour of #84