perf(formatting numbers): stop using a default locale, use the system one

When no local provided, it will use the user one.

It should be more relevant AND does provide a good perf boost.

Author: vvo

src/lib/InstantSearch.js

@@ -40,7 +40,7 @@ class InstantSearch extends EventEmitter {
     appId = null,
     apiKey = null,
     indexName = null,
-    numberLocale = 'en-EN',
+    numberLocale,
     searchParameters = {},
     urlSync = null
   }) {