Use Symbol for default document loader key.

Author: davidlehn

lib/jsonld.js

@@ -105,7 +105,7 @@ const RESOLVED_CONTEXT_CACHE_MAX_SIZE = 100;
 // resources are cleaned up with WeakMap semantics for the documentLoaders
 const _resolvedContextCaches = new WeakMap();
 // default key to use when no documentLoader used
-const _defaultDocumentLoaderKey = {};
+const _defaultDocumentLoaderKey = Symbol();
 
 // make a ContextResolver using a per-documentLoader shared cache
 function _makeContextResolver({documentLoader = _defaultDocumentLoaderKey}) {