fix: make fields with index signatures a dynamic mapping

src/mapping.ts

@@ -222,6 +222,8 @@ function handleDeclarationReflection(decl: DeclarationReflection,
   let empty = true;
   // first check if there are any index signatures, so for example `[name: string]: Foo`
   if (typeof decl.indexSignature !== 'undefined' && typeof decl.indexSignature.parameters !== 'undefined') {
+    out.dynamic = true;
+
     for (const param of decl.indexSignature.parameters) {
       empty = false;
       const template: ElasticsearchDynamicTemplate = {};