Enable closing after "keepOpen()" has been called

[martypdx]

Test and changes to request to allow .keepOpen() request to actually close, fixes #189.

FYI Side Note - I had to disable posttest script during local development as it causes npm test to fail (I have no token) with:

> if [ -z "$COVERALLS_REPO_TOKEN" ]; then cat coverage/lcov.info | coveralls; fi

/Users/marty/dev/chai-http/node_modules/coveralls/bin/coveralls.js:18
        throw err;
        ^
Bad response: 422 {"message":"Couldn't find a repository matching this job.","error":true}

[keithamus]

Great work! I approve of this. Someone else from @chaijs/chai-http should merge!

[JamesMessinger]

LGTM