http: default Agent.getName to 'localhost'
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 <6b79c807f33c79ce9edd01e7fc5cbf832e85609c@gmail.com>
Reviewed-By: Rich Trott <28a7b6144c43947e3f532c033ff42c8a2ae08045@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node