fix($http): apply transformResponse when data is empty (for non-HEAD requests)

[gkalpak]

Fixes #12976

[gkalpak]

Possible BREAKING CHANGE:

Previously, non-empty responses to HEAD requests were transformed' not any more.
I didn't consider this a BC though, because it was never explicitly supported and according to RFC2616:
"The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response."

We could as well change it so that even HEAD responses are transformed, if we feel more comfortable :)

[Narretz]

LGTM. Don't think it's a BC. @petebacondarwin wdyt?

[petebacondarwin]

I agree. It is not a BC.
Moreover, I am inclined to agree that we should just transform everything. Although HEAD requests are not "supposed" to return any data, there are, no doubt, servers out there that do. What do we care if they return data or not? We should just transform everything but not crash if the data is empty.

[gkalpak]

Updated the commit. Now we always trasform the data.

<sub>It just occurred to me:
If someone were using their own tranform function(s) (not the default), and were operating under the assumption that the data wouldn't be empty, they will experience some "broken-ness" :)
FWIW, it isn't mentioned or implied anywhere that data passed to the transform callback won't be empty, but that's how it was.

I still don't think this should be documented as a BC; just pointing it out.</sub>

@petebacondarwin, @Narretz: PTYL.

[petebacondarwin]

To be on the safe side, let's ensure that the commit documents this change, highlighting the case where someone's code may break if they were relying on the "undocumented" behaviour.

[gkalpak]

I updated the commit message.

_{Just to make sure I interpreted your comment correctly, @petebacondarwin:
You didn't mean I should ad a BC notice, did you ?}