HTTP KeepAlive Fixes + Drop Support for Go 1.7

leaktest now supports only Go 1.8 and beyond. If you need earlier support, use v1.2.0

Some keepalive fixes from @lawrencegripper

1.2.0

Few small PRs (IIRC), most notably, CheckTimeout works as expected in this version, thanks to @ecnerwala and @seglberg

Go 1.7+

This release is the first release to support CheckTimeout and CheckContext.

Pre 1.7 Support Edition

I'm tagging v1.0.0 from current master prior to merging of #12 so that anyone stuck on Go 1.5 / 1.6 can still use leaktest by pinning to this tag