fix(twilio-run): correctly serialize JSON Responses

[philnash]

Fixes #276

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[philnash]

@dkundel This probably needs porting into the runtime-handler too.

[dkundel]

Yes can we merge the runtime-handler first and then push this change? It's just easier to keep track of it that way.

[philnash]

@dkundel Fixed this in runtime-handler too.