respond.Before unable to find original request with Go1.7+ native Context

[baloo32]

Due to the changes in Go1.7+ and the implementation of a native Context, the respond.Before operation fails due to the original request being duplicated within WithContext and not available to the Before handler.

Example:

func (r *Request) WithContext(ctx context.Context) *Request {
    if ctx == nil {
        panic("nil context")
    }

    r2 := new(Request)
    *r2 = *r
    r2.ctx = ctx
    return r2
}

This sample code will not be able to access the original data object to resolve the .Public interface:

responder.Before = func(w http.ResponseWriter, r *http.Request, status int, data interface{}) (int, interface{}) {
        if err, ok := data.(error); ok {
            return status, map[string]interface{}{"error": err.Error()}
        }
        if obj, ok := data.(interface {
            Public() interface{}
        }); ok {
            return status, obj.Public()
        }
        return status, data
    }