Fix buildMethodDesc for two and more message field (#1979)

cmd/protoc-gen-go-http/http.go

@@ -163,9 +163,10 @@ func buildMethodDesc(g *protogen.GeneratedFile, m *protogen.Method, method, path
 	defer func() { methodSets[m.GoName]++ }()
 
 	vars := buildPathVars(path)
-	fields := m.Input.Desc.Fields()
 
 	for v, s := range vars {
+		fields := m.Input.Desc.Fields()
+
 		if s != nil {
 			path = replacePath(v, *s, path)
 		}