Allow StdErrSplitter to conform to full $stderr interface

These changes are required to support capturing the `$stderr` stream in a tempfile rather than a `StringIO`. See https://github.com/rspec/rspec-expectations/pull/700 for the rationale.
This spec is failing on windows because of a weird problem with `close_on_exec` that I don't really understand.  Is there one of the methods that we can use as a representative of this category of method rather than all of them?  Then you could actually call it (since the fix was the change to `method_missing`, not to `respond_to?`).

Or we could just delete this spec if that's easier.  Since this class is only used in RSpec's spec suite I'm not terribly concerned about this.