Add support for getting headers to ClientRequest

This is necessary for things like signed requests.

Author: thomdixon

client_request.go

@@ -17,6 +17,7 @@ package runtime
 import (
 	"io"
 	"io/ioutil"
+	"net/http"
 	"net/url"
 	"time"
 
@@ -40,6 +41,8 @@ type ClientRequestWriter interface {
 // add information to a swagger client request
 type ClientRequest interface {
 	SetHeaderParam(string, ...string) error
+	
+	GetHeaderParams() http.Header
 
 	SetQueryParam(string, ...string) error