Introduce `srvTerminateWithResponse`

The `ServerAPI` functions `srvWriteBuilder`, `srvSetContentType` and
`srvSetResponseCode` have been merged into a single function:
`srvTerminateWithResponse`. This should allow `ServerAPI` to be provided
for Yesod.
engine-io-snap: Update shell.nix
engine-io-snap: Fix incomplete pattern matches