http: use defaultAgent.protocol in protocol check

Default to the `defaultAgent.protocol` when comparing the
user-specified `options.protocol` string. This is so that
`http.Agent` instances do not need to specify their own
`protocol` field, since we have the relevant information
already from the `defaultAgent`.

Note that the test case could be separately cherry-picked
to the `v0.10` branch, since it already passes correctly.

Fixes #7349.
Fixes the regression described in: http://git.io/2ds-WQ