remove dependency on gorilla/context for go1.7+

[mna]

Hello,

This finishes what was started a few commits ago by moving the use of gorilla/context behind a build tag, so that for Go1.7+ that package isn't used anymore. It adds the missing contextClear func, and uses the contextSave call to set the error in the context while making sure to change the *http.Request to the one returned by contextSave.

Thanks,
Martin

[elithrar]

Good catch—thanks for this @PuerkitoBio!