Expose swappedResponseWriter

[bscottm]

Respectfully request that you expose swappedResponseWriter for better compatibility with authBoss. This might not be the only package with this type of issue.

In their design, they expect a http.Writer of a particular type (ClientStateResponseWriter) when writing responses. It's relatively easy to work around if swappedResponseWriter is exposed as SwappedResponseWriter (currently I have a local copy of adapter.go to make things work...):

import (
    // ...
    _ "github.com/volatiletech/authboss/v3"
)

func (w swappedResponseWriter) UnderlyingResponseWriter() http.ResponseWriter {
	return w.w
}