Fix bug when using shared output object

[ejizba]

Fixes #179

Repro steps:

1. Have two functions triggered by the same thing
2. Create an output binding as a const
3. Use the same output binding as the return in one function and as an extraOutput in the other

Expected: Both output bindings work
Actual: The return output binding works and the extraOutput one doesn't set the output

End-to-end test (with sample code) here: Azure/azure-functions-nodejs-e2e-tests#30