Adjust a double leading slash in the path for FTP to /%2F

Adjust a double leading slash in the path for FTP to /%2F

Some FTP implementations (currently not including QNAM) strip the first
slash off the path in an FTP URL so that the path in the URL is relative
to the login path (the user's home directory). To reach the root
directory, another slash is necessary, hence the double slash.

In anticipation of future URL normalisation, which Qt 4 could do, "//"
could be rendered to "/", so this extra slash should be "%2F".

This operation is done only in QUrl::fromUserInput.

Change-Id: If9619ef6b546a3f4026cb26b74a7a5a865123609
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>