http: default Agent.getName to 'localhost'

Refactor out the if/else statement checking for option.host.
Add whitespace to make concatenation chunks more readable and
consistent with the https version of Agent.getName().

PR-URL: https://github.com/nodejs/node/pull/2825


Reviewed-By: Julian Duque <julianduquej@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>