fix($http): do not modify the config object passed into $http short methods

[maxthyen]

Update $http's createShortMethods and createShortMethodsWithData to extend an empty object instead of the passed-in config.

Previously, since $http was extending the passed-in config, the changes to the config object persisted even after the call to $http's get/post/etc. returned:

var config = { some_key: 1 };
$http.get('/testUrl', config);
// config now contains { some_key: 1, url: '/testUrl', method: 'GET' }

This causes unexpected behavior if that config object is reused in subsequent calls to $http. The existing test in httpSpec was not properly testing this situation.

[johnculviner]

👍 this caused a pretty lengthy head scratching session for @maxthyen and I until the behavior was realized. Can't think of any reasons why this would be desirable?

[matthewdordal]

👍 being able to reuse a config object without it being changed under the hood is ideal.

[lgalfaso]

I like this PR, just wondering if anybody is relying on this odd behavior to think that this is a breaking change.
@petebacondarwin WDYT?

[petebacondarwin]

I agree. It looks good. We should merge it now and label it as a small breaking change - just in case.

[petebacondarwin]

Do you want to take that @lgalfaso ?

[lgalfaso]

landed as f7a4b48