Fix with_isolated_stderr so it does not leak.

[myronmarston]

Without ensure, it leaves $stderr set to a different value.

Fixes rspec/rspec-core#1534.

So that rspec-core issue wasn't actually a problem with rspec-core; it was a problem with this line in an rspec-mocks test:

https://github.com/rspec/rspec-mocks/blob/83c1456d9e9bddcb624b00f636340a837f42e6b2/spec/rspec/mocks/argument_matchers_spec.rb#L209

...which used with_isolated_stderr, was wrapping a spot where an exception was raised, and as a result $stderr would remain silenced after that. That in turn cased the exception in the formatter not to be printed to the console like it normally would be.

/cc @JonRowe

[cupakromer]

LGTM

[JonRowe]

Makes sense, nice simple change too, possibly worth a bugfix mention on core somewhere?

[myronmarston]

Nope. with_isolated_stderr isn't used by any of the lib code of any of our projects -- it's just a spec helper method that affected our specs. There's no way for a user to have been affected by this.

[JonRowe]

Oh for some reason I thought this was part of the stderr matcher, my bad :)