lib: adding keep-alive header to download requests
lib: adding keep-alive header to download requests

PR-URL: https://github.com/nodejs/node-gyp/pull/1863
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>

nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp