Allow engine root relative redirects using an empty string.

Example:

    # application routes.rb
    mount BlogEngine => '/blog'

    # engine routes.rb
    get '/welcome' => redirect('')

This now redirects to the path `/blog`, whereas before it would redirect
to the application root path. In the case of a path redirect or a custom
redirect if the path returned contains a host then the path is treated as
absolute. Similarly for option redirects, if the options hash returned
contains a `:host` or `:domain` key then the path is treated as absolute.

Fixes #7977