func (t *RequestModifyingTransport) RoundTrip(req *http.Request) (resp *http.Response, err error) {
	req = t.applyOverrides(req)
	transport := t.Transport
	if transport == nil {
		transport = http.DefaultTransport
	}
	return transport.RoundTrip(req)
}
RoundTrip implements net/http.RoundTripper.
package apiproxy
import (
	"net/http"
	"net/textproto"
	"regexp"
	"strings"
	"sync"
)
type RequestModifyingTransport struct {
	overrides   map[*regexp.Regexp]requestOverride
	overridesMu sync.Mutex
	Transport http.RoundTripper
}