Fix delegator return value for single hijacker
Fix delegator return value for single hijacker

Previously, the pickDelegator function was not returning a
*hijackerDelegator so the return value did not implement the Hijacker
interface. As a result, code that attempts to hijack the connection
would fail when using a type assertion.

All the other cases returned the hijackerDelegator correctly.

prometheus/client_golang,m3db/prometheus_client_golang