fix($browser): prevent infinite digest if changing hash when there is no hashPrefix

The `window.location.hash' setter will strip of a single leading hash character
if it exists from the fragment  before joining the fragment value to the href
with a new hash character.

This meant that if we want the fragment to lead with a hash character, we
must do `window.location.hash = '##test'` to ensure that the first hash
character in the fragment is not lost.

The `$location.hash` setter works differently and assumes that the value
passed is the the full fragment, i.e. it does not attempt to strip off a
single leading hash character.

Previously, if you called, `$location.hash('#test')`, the leading hash was
being stripped and the resulting url fragment did not contain this hash:
`$location.hash()`, then became 'test' rather than `#test`, which led to
an infinite digest.

Closes #10423
Closes #12145