proxy: keep all query string parameters, but "login"

Note that the parameters are not encoded.
proxy - Use URIBuilder to create the query string

Note that it encodes the URI, eg. if a parameter is "fré to", it
will be encoded as "fré+to". This behavior will happen only when a
"login" parameter is present in the URI. In other cases, the parameter
stays unchanged.
Merge pull request #480 from georchestra/issue477

proxy: keep all query string parameters, but "login" - fixes #477