Fix including http:// in the proxy URL
When proxy was provided with a scheme, the proxy host parsed incorrectly.
Now its allowed to specify proxy with the scheme

Fix invalid type conversion
Calling `proxyPort()` in string concatenation caused to cast it to its ASCII representation.
Use `QString::number` to include a proxy port properly.

Issues:
https://github.com/ariya/phantomjs/issues/11117
https://github.com/ariya/phantomjs/issues/10811