AngularJS: fixed typing of the ng.IPromise.then function

Hello,

I apologize in advance for any potential blunders, I'm new to github and community projects.
Commit message is below, it explains the problem that I'm trying to address.
Thanks for reviewing!

-a


The type definition for ng.IPromise.then was, in my opinion, incorrect
semantically, and breaking on practical examples in my codebase.

An `IPromise`' `then` function takes a callback which is called with the
value of the promise once it's fulfilled. This could be a number, a
string, some object, an array of strings, anything really. Yet the
typing in angular.d.ts specified `then` as taking a `successCallback` of
type `(response: PromiseCallbackArg) => any`.

The definition of `PromiseCallbackArg` seems very permissive at first
glance, as it is defined to be an object with a bunch of fields, all
optional. Any object, a number, and a string all type check correctly
with such a type, but an array of strings, for example, does not
(`cPromise` in the provided list of test).

Furthermore, it seems to me that the `PromiseCallbackArg` definition was
added specifically to support the response type for promises returned by
the Angular `$http` service, the `ng.IHttpPromise`.

So instead of having an incorrect type on the `ng.IPromise.then` function,
I propose we return it to its generic form, and instead override the
type of the inherited `then` function in the `ng.IHttpPromise` interface.
This would also warrant renaming `PromiseCallbackArg` to
`IHttpPromiseCallbackArg`.