fix($http): won't parse single space response

[gampleman]

According to this question, Rails in order to patch a bug in Safari, returns a single space when calling head :ok, which is the standard for POST requests.

Angular since 9ba24c54d checks if the body is empty before calling JSON parse, but this still breaks in this case, since the body isn't empty, but contains a single space.

We are hitting this issue when trying to upgrade from Angular 1.2.10. This PR should help in that case.

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[caitp]

I think this doesn't actually make sense, since '' is still falsy

[pkozlowski-opensource]

@gampleman thnx for this PR but I don't think we can merge it as-is - see my comments in the code.

[pkozlowski-opensource]

So while it kind of make sense that we trim strings before running it through toJson people could easily work-arround it already by adding a custom transformer so I guess it is not critically urgent that we get this fix in.

[gampleman]

OK, so I've changed it to check right before attempting to parse the JSON that the response isn't just blank.

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Side note: For us it's a blocker on upgrading to Angular 1.3, since all our APIs have this behaviour. We don't want to change Rails and while adding a custom transformer everywhere is an option, it doesn't seem too attractive.

[pkozlowski-opensource]

LGTM, merging