angular-resource.d.ts. parameter of IResource<T> change to optional

@miffels @scottmcarthur I want to change parameter of IResource<T> change to optional.
Can I merge this PR?

this PR is not beautiful. but I think this hack is useful.
https://github.com/Microsoft/TypeScript/issues/371
If this proposal are accept by MS, this PR is unnecessary :)
I want succeed the compilation of this line.