[#8B0487D9FBD591BF3D4E4663ECA58B192C9CE2C0]
Return true a scheme is present
                            
        
            bool
has_scheme() const noexcept;
        
    This function returns true if this contains a scheme.
assert( url_view( "http://www.example.com" ).has_scheme() );
Constant.
Throws nothing.
URI             = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
absolute-URI    = scheme ":" hier-part [ "?" query ]
scheme          = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )