Add '=' after '_h' for Horde_Url objects in signUrl()/signQueryString()

[amulet1]

The horde/Url@bf2b51b restored the original behavior or Horde_Url ('=' is not added for parameters with empty values).

See #28 (comment).

[amulet1]

@TDannhauer Please merge.

[TDannhauer]

What symptoms fixes this change?

[amulet1]

It fixes horde/base#12 (again).

Here is what happened:

1. Commit horde/Url@523d5e5b introduced multiple issues (Horde_Url no longer passing tests, '=' added for empty parameter values, not converting Horde_Url objects to string, etc.).
2. The Remove extraneous '=' in signUrl() and signQueryString() #28 fixed the '=' part of it in order to close Messed up URL when redirecting to auth during login or refresh base#12.
3. Reimplement parameters encoding in Horde_Url's toString() method to restore the intended functionality Url#4 (just merged) fixed all other broken things, but it also restored the original handling of empty values (i.e. does not add '=').
4. That is why we now need to add back '='.