Make reverse proxy auth optional (#4643)

* Make reverse proxy auth optional

If the option ENABLE_REVERSE_PROXY_AUTHENTICATION is enabled, make
reverse proxy auth optional, instead of failing if the authentication
did not succeed.

Fixes #3973

Signed-off-by: Najib Idrissi <najib.idrissi.kaitouni@gmail.com>

* Update http.go

routers/repo/http.go

@@ -83,12 +83,8 @@ func HTTP(ctx *context.Context) {
 
 	// check access
 	if askAuth {
-		if setting.Service.EnableReverseProxyAuth {
 		authUsername = ctx.Req.Header.Get(setting.ReverseProxyAuthUser)
-			if len(authUsername) == 0 {
-				ctx.HandleText(401, "reverse proxy login error. authUsername empty")
-				return
-			}
+		if setting.Service.EnableReverseProxyAuth && len(authUsername) > 0 {
 			authUser, err = models.GetUserByName(authUsername)
 			if err != nil {
 				ctx.HandleText(401, "reverse proxy login error, got error while running GetUserByName")