set options on both agent and `request` (fix #98)
set options on both agent and `request` (fix #98)

* ensure keep-alive is enabled
* create agent lazily
* only create one HTTP agent and one HTTPS agent
* only pass HTTPS parameters to HTTPS agent

npm/npm-registry-client,teddyhwang/npm-registry-client,smikes/npm-registry-client,seegno-forks/npm-registry-client