Contact Form: Submit location must respect current scheme

Using Jetpack with [WordPress HTTPS (SSL)](https://github.com/Mvied/wordpress-https), I got an warning from User-agent(Chrome).
> The page at 'https://www.extendwings.com/contact/en/' was loaded over HTTPS, but is submitting data to an insecure location at 'http://www.extendwings.com/contact/en/#contact-form-3142': this content should also be submitted over HTTPS.

I think using relative reference(defined at [RFC 3986 Section 4.2](http://tools.ietf.org/html/rfc3986#section-4.2)) for scheme is good.

So, this remove scheme from URL.