Merge pull request golang#591 from golang/master-merge

Merge remote-tracking branch 'origin/dev' into master

proto: add DiscardUnknown function (golang#498)

This copies the discardLegacy logic from the dev branch.
The unit tests are located on the dev branch and will be merged
into the master branch in the near future.

This function is cherrypicked into master in order to support the
master to dev transition.