refactor($http): avoid re-creating execHeaders function

[pkozlowski-opensource]

The execHeaders function was being re-defined inside mergeHeaders
function. Additionally it was mutating its arguments thus making code
harder to read / reason about.

There are some other problems with this function (inconsistent treatment for null / undefined headers coming from an object value as compared to being result of a function execution) but let's tackle those one by one.

[googlebot]

CLAs look good, thanks!

[caitp]

+1 for not needing this

[caitp]

I don't think this is likely to break anyone, and it is a bit simpler without worrying about the "delete" case --- Not too worried about perf implications since it's just $http, and delete is likely to be slower anyways... so basically LGTM. I'd prefer if the executeHeaderFns were defined lower in the source though, (eg below mergeHeaders)

[pkozlowski-opensource]

@caitp I see what you mean by pushing it down in the code but for me this refactoring was enabler for other refactorings. Basically what I would like to do is to push those functions out of each $http call - today those are re-created on each call to $http and there is no good reason for doing it IMO. But yeh, this will come with a separate PR

[caitp]

In fact, they are not actually "re-created" in any engine I'm aware of --- apart from subsequent optimization which can happen in subsequent calls, it doesn't really do anything --- my LGTM is mostly about the getting rid of the "delete" behaviour.