http: cleanup setHeader()

Several fields on OutgoingMessage were set after instantiation. These
have been included in the constructor to prevent mutation of the object
map after instantiation.

"name" is now explicitly checked to be a string. Where before if a
non-string was passed the following cryptic error was thrown:

    _http_outgoing.js:334
      var key = name.toLowerCase();
                     ^
    TypeError: undefined is not a function

Signed-off-by: Trevor Norris <trev.norris@gmail.com>