only supply the id attribute to knex.insert for drivers that support it

Knex 0.16.4 began warning when the returning attribute is supplied for
dialects that do not support it.  These dialects currently include MySQL
and SQLite3.  Supplying this attribute is a no-op for those dialects, so
excluding them is safe and eliminates the warning.

More information:

    knex/knex#3039

src/index.js

@@ -421,6 +421,13 @@ function loadWithRelations (items, resourceConfig, options) {
 Adapter.extend({
   constructor: SqlAdapter,
 
+  _returning (idAttribute) {
+    // Some knex client drivers do not support .returning(), so set the
+    // attribute to null to prevent knex from emitting warnings.
+
+    return ['mysql', 'sqlite3'].includes(this.knexOpts.client) ? null : idAttribute
+  },
+
   _count (mapper, query, opts) {
     opts || (opts = {})
     query || (query = {})
@@ -438,7 +445,7 @@ Adapter.extend({
 
     const sqlBuilder = utils.isUndefined(opts.transaction) ? this.knex : opts.transaction
     return sqlBuilder(getTable(mapper))
-      .insert(props, idAttribute)
+      .insert(props, this._returning(idAttribute))
       .then((ids) => {
         const id = utils.isUndefined(props[idAttribute]) ? (ids.length ? ids[0] : undefined) : props[idAttribute]
         if (utils.isUndefined(id)) {