Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
@myronmarston That's my experience thus far. I have been unable to cause the code to fail with that exception, which is why I tracked down where it came from.  Specifically, as I noted in the commit message https://github.com/bf4/rspec-support/commit/96584ba8247014c81aa0031ca461dfed0589c561

```plain
The Differ no longer catches CompatibilityErrors
This was introduced in
https://github.com/rspec/rspec-expectations/pull/220/files#diff-9533f5f156a38a3307ecfc610d2282d7R47

but is no longer raised either by the the current test code, the
original test code
      @expected="Tu avec carté {count} itém has".encode('UTF-16LE')
      @actual="Tu avec carte {count} item has".encode('UTF-16LE')
or any other variations I've tried

Since I'm moving all the encoding logic into EncodedString, and this
doesn't appear to do anything any more, I am proposing to remove it.
```

As I commented elsewhere, I want to confirm this isn't still a problem on 1.9.2.