Fixed Unicode issue by encoding $transcript to UTF-8

This is one-line fix that encodes `$transcript` to UTF-8 so that non-ASCII characters are not interperted as garbage. Tested with Asterisk 1.8.22.0. With this fix, I can successfully give `${utterance}` to `googletts.agi` without any encoding issues.
Merge pull request #15 from Zopieux/patch-1

Fixed Unicode issue by encoding $transcript to UTF-8