revert: fix($location): return '/' for root path in hashbang mode

This reverts commit 63cd873fef3207deef30c7a7ed66f4b8f647dc12.

The change breaks existing tests of Google apps. The problem is that
while we tried to avoid adding #/ to window.location.href unnecessarily
we failed doing so. Likely because by setting $path, at some point
(during a digest) we try to check if $location changed and we mistake the
default '/' with an explicit settign of the path via the `path()` method.
This results in us writing the url with '#/' into $browser.url() which updates
the window.location by adding "#/" to the url - something we tried to avoid
in the first place.

I'll reopen PR #5712.