Remove extraneous '=' in signUrl() and signQueryString()

[amulet1]

The verification code in verifySignedUrl and verifySignedQueryString methods expects the $data string to be in the following format: {url_with_timestamp}&_h={hash}. The supplied hash is checked against the hash computed on {url_with_timestamp}&_h= part.

However, both signUrl and signQueryString add an extra = (only in case if the supplied parameter is an instance of Horde_Url class), i.e. a hash is computed on {url_with_timestamp}&_h==, and the subsequent verification fails.

This is required (but not enough) in order to resolve horde/base#12

[amulet1]

@ralflang, please review.