Auto merge of #6668 - eregon:fix-encoding-spec-from-6661, r=deivid-rodriguez
Auto merge of #6668 - eregon:fix-encoding-spec-from-6661, r=deivid-rodriguez

Fix encoding spec from #6661 to also work 1.9.3

* String#b is Ruby 2.0+.

### What was the problem that led to this PR?

The CI is failing on the `1-16-stable` branch.

### What was your diagnosis of the problem?

The CI on master doesn't run against MRI 1.9.3, so this failure slipped in in #6661.
Probably testing against MRI 1.9.3 should be done on `master` too?

### What is your fix for the problem, implemented in this PR?

Make the spec work on 1.9.3.

cc @hsbt

bundler/bundler,dilumnavanjana/bundler,mvz/bundler,dilumnavanjana/bundler,mvz/bundler,bundler/bundler