Set the request body to a new ReadCloser

caddyhttp/httpserver/replacer.go

 package httpserver
 
 import (
+	"bytes"
 	"io/ioutil"
 	"log"
 	"net"
@@ -127,6 +128,9 @@ func NewReplacer(r *http.Request, rr *ResponseRecorder, emptyValue string) Repla
 				}
 
 				body, err := ioutil.ReadAll(r.Body)
+				// Create a new ReadCloser to keep the body from being drained.
+				r.Body = ioutil.NopCloser(bytes.NewBuffer(body))
+
 				if err != nil {
 					log.Printf("[WARNING] Cannot read request body %v", err)
 					return ""