fix(ngResource): correct leading slash removal.

Fixed an issues with ngResource param substitution where it was incorrectly removing leading slash when param was followed by a non-slash character.
Ex:
'/:foo/:bar.baz/:aux'

params = {
  foo: 'aaa',
  bar: undefined,
  aux: undefined
}

The above params were incorrectly producing '/aaa.baz' but now it results in '/aaa/.baz'.