Convert keywords to string before setting state.
Convert keywords to string before setting state.

Fixes https://github.ibm.com/Watson/developer-experience/issues/1386
Fixes comment on https://github.ibm.com/Watson/developer-experience/issues/1393

leonrch/speech-to-text-nodejs,watson-developer-cloud/speech-to-text-nodejs,watson-developer-cloud/speech-to-text-nodejs