http: don't confuse automatic headers for others

If you set a custom http header which includes eg. the string `Date`,
then http will not automatically send the `Date` header.

This is also true for other automatic http headers.

PR-URL: https://github.com/iojs/io.js/pull/828


Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>