fix: Provide correct return value from source.emit() (#17)

This rebases the work from #8 with minor modifications, and tests the return values directly (rather than adding a wide-bracket integration test).

Adopts arrow functions in the test file.

This was identified as the root cause of nock/nock#1485.

Thanks to @rbrtribeiro for the fix! 

Close #8 Close #9

chore(test): Bump tap from 0.3.x to 12.6.1 and generate coverage repo…

…rts (#11)