Support Requests directly from trusted proxy

When returning the protocol used handle the case where the trusted proxy has made a request to a express directly.  When this occurs use the encrypted state on the underlying connection.

This happens with Amazon's ELBs with auto-scaling groups.  The ELB will ping and healthcheck endpoint on the node server to verify its health.  In this case there is no X-Forwarded-Proto header since the request originates directly from the ELB.