Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Is this being removed because of https://github.com/rspec/rspec-expectations/pull/220/files#diff-9533f5f156a38a3307ecfc610d2282d7R47 ?

I ask because rspec-mocks uses the differ in RSpec 2.2....but it doesn't rescue this error.  It makes me wonder if we should move this rescue back into here and/or add a similar rescue to rspec-mocks.

@JonRowe, give that you authored rspec/rspec-expectations#220, what do you think should be done?