fix: deep copy imported contexts

Author: jeswr

lib/ContextParser.ts

@@ -816,13 +816,13 @@ must be one of ${Util.CONTAINERS.join(', ')}`, ERROR_CODES.INVALID_CONTAINER_MAP
     // First try to retrieve the context from cache
     const cached = this.documentCache[url];
     if (cached) {
-      return typeof cached === 'string' ? cached : Array.isArray(cached) ? cached.slice() : {... cached};
+      return typeof cached === 'string' ? cached : JSON.parse(JSON.stringify(cached));
     }
 
     // If not in cache, load it
     let document: IJsonLdContext;
     try {
-      document = await this.documentLoader.load(url);
+      document = JSON.parse(JSON.stringify(await this.documentLoader.load(url)));
     } catch (e) {
       throw new ErrorCoded(`Failed to load remote context ${url}: ${e.message}`,
         ERROR_CODES.LOADING_REMOTE_CONTEXT_FAILED);