Provide wrapper method for new StringIO objects

Previously, StringIO objects were being created directly from
the provider methods. This worked just fine, except for the
fact that the StringIO objects were identified by a serial number.
When you would try to provide rspec test coverage around these
provider methods, you could never provide a complete match for
the StringIO objects as you would first need to mock StringIO.new,
and then hope that the serial numbers matched.

This commit wraps the StringIO.new method into its own provider
method called 'new_stringio_object', which makes it easier to mock
for rspec tests.