Add REQUEST_SCHEME to fastcgi envs

Fixes https://github.com/mholt/caddy/issues/2152

caddyhttp/fastcgi/fastcgi.go

@@ -285,6 +285,11 @@ func (h Handler) buildEnv(r *http.Request, rule Rule, fpath string) (map[string]
 	// Retrieve name of remote user that was set by some downstream middleware such as basicauth.
 	remoteUser, _ := r.Context().Value(httpserver.RemoteUserCtxKey).(string)
 
+	requestScheme := "http"
+	if r.TLS != nil {
+		requestScheme = "https"
+	}
+
 	// Some variables are unused but cleared explicitly to prevent
 	// the parent environment from interfering.
 	env = map[string]string{
@@ -301,6 +306,7 @@ func (h Handler) buildEnv(r *http.Request, rule Rule, fpath string) (map[string]
 		"REMOTE_IDENT":      "", // Not used
 		"REMOTE_USER":       remoteUser,
 		"REQUEST_METHOD":    r.Method,
+		"REQUEST_SCHEME":    requestScheme,
 		"SERVER_NAME":       h.ServerName,
 		"SERVER_PORT":       h.ServerPort,
 		"SERVER_PROTOCOL":   r.Proto,

caddyhttp/fastcgi/fastcgi_test.go

@@ -248,6 +248,12 @@ func TestBuildEnv(t *testing.T) {
 	envExpected["SCRIPT_NAME"] = "/fgci_test.php"
 	envExpected["SCRIPT_FILENAME"] = filepath.FromSlash("/fgci_test.php")
 	testBuildEnv(r, rule, fpath, envExpected)
+
+	// 8. Test REQUEST_SCHEME in env
+	r = newReq()
+	envExpected = newEnv()
+	envExpected["REQUEST_SCHEME"] = "http"
+	testBuildEnv(r, rule, fpath, envExpected)
 }
 
 func TestReadTimeout(t *testing.T) {