No unicode + StringIO in tests

This change alters some tests so that unicode is not used with StringIO objects. This causes exceptions in the test suite.

This issue seems to be fixed in Python 2.7.3 [1], but is still an issue for other 2.7 versions. (For example, OSX 10.7 ships with 2.7.1.) This change simply makes the tests more fault tolerant in more diverse
environments.

[1] - http://hg.python.org/cpython/file/d46c1973d3c4/Misc/NEWS#l300