fix(agent): do not throw error on circular dependencies

Author: dszakallas

package.json

@@ -45,6 +45,7 @@
     "continuation-local-storage": "3.2.0",
     "debug": "2.2.0",
     "https-proxy-agent": "1.0.0",
+    "json-stringify-safe": "5.0.1",
     "lodash.assign": "4.2.0",
     "lodash.defaults": "4.2.0",
     "lodash.find": "4.6.0",