Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/pagination/http.go b/pagination/http.go
index d3bd490..ba2c615 100644
--- a/pagination/http.go
+++ b/pagination/http.go
@@ -39,8 +39,8 @@
 
 	return PageResult{
 		Result: gophercloud.Result{
-			Body:    parsedBody,
-			Headers: resp.Header,
+			Body:   parsedBody,
+			Header: resp.Header,
 		},
 		URL: *resp.Request.URL,
 	}, err