http: Only send connection:keep-alive if necessary

In cases where the Agent has maxSockets=Infinity, and
keepAlive=false, there's no case where we won't immediately close the
connection after the response is completed.

Since we're going to close it anyway, send a `connection:close` header
rather than a `connection:keep-alive` header.  Still send the
`connection:keep-alive` if the agent will actually reuse the socket,
however.

Closes #5838