changes as noted in PR

middleware/proxy/reverseproxy.go

@@ -16,8 +16,6 @@ import (
 	"time"
 )
 
-const HTTPSwitchingProtocols = 101
-
 // onExitFlushLoop is a callback set by tests to detect the state of the
 // flushLoop() goroutine.
 var onExitFlushLoop func()
@@ -149,13 +147,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request, extr
 	}
 	defer res.Body.Close()
 
-	for _, h := range hopHeaders {
-		res.Header.Del(h)
-	}
-
-	copyHeader(rw.Header(), res.Header)
-
-	if res.StatusCode == HTTPSwitchingProtocols && outreq.Header.Get("Upgrade") == "websocket" {
+	if res.StatusCode == http.StatusSwitchingProtocols && outreq.Header.Get("Upgrade") == "websocket" {
 		hj, ok := rw.(http.Hijacker)
 		if !ok {
 			return nil
@@ -182,6 +174,12 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request, extr
 		io.Copy(conn, backendConn) // read tcp stream from backend.
 		conn.Close()
 	} else {
+		for _, h := range hopHeaders {
+			res.Header.Del(h)
+		}
+
+		copyHeader(rw.Header(), res.Header)
+
 		rw.WriteHeader(res.StatusCode)
 		p.copyResponse(rw, res.Body)
 	}

middleware/proxy/upstream.go

@@ -14,7 +14,7 @@ import (
 
 var (
 	supportedPolicies map[string]func() Policy = make(map[string]func() Policy)
-	proxyHeaders      http.Header
+	proxyHeaders      http.Header              = make(http.Header)
 )
 
 type staticUpstream struct {
@@ -100,10 +100,10 @@ func NewStaticUpstreams(c parse.Dispenser) ([]Upstream, error) {
 				if !c.Args(&header, &value) {
 					return upstreams, c.ArgErr()
 				}
-				addProxyHeader(header, value)
+				proxyHeaders.Add(header, value)
 			case "websocket":
-				addProxyHeader("Connection", "{>Connection}")
-				addProxyHeader("Upgrade", "{>Upgrade}")
+				proxyHeaders.Add("Connection", "{>Connection}")
+				proxyHeaders.Add("Upgrade", "{>Upgrade}")
 			}
 		}
 
@@ -153,14 +153,6 @@ func RegisterPolicy(name string, policy func() Policy) {
 	supportedPolicies[name] = policy
 }
 
-// AddProxyHeader adds a proxy header.
-func addProxyHeader(header, value string) {
-	if proxyHeaders == nil {
-		proxyHeaders = make(map[string][]string)
-	}
-	proxyHeaders.Add(header, value)
-}
-
 func (u *staticUpstream) From() string {
 	return u.from
 }