Avoid header param mutation.

Author: davidlehn

lib/documentLoaders/node.js

@@ -40,9 +40,11 @@ module.exports = ({
   httpsAgent
 } = {strictSSL: true, maxRedirects: -1, headers: {}}) => {
   headers = buildHeaders(headers);
-  // set a default header if none was set
+  // if no default user-agent header, copy headers and set one
   if(!('user-agent' in headers)) {
-    headers['user-agent'] = 'jsonld.js';
+    headers = Object.assign({}, headers, {
+      'user-agent': 'jsonld.js'
+    });
   }
   const http = require('http');