Fix unicode_raises_error test

One of the WSGI tests changes ``HttpProtocol`` class member ``minimum_chunk_size`` and leaves it like that, what in turn causes ``test_unicode_raises_error`` to fail in some circumstances (first chunk of data is processed successfully so response_line check fails):

```
======================================================================
FAIL: test_unicode_raises_error (tests.wsgi_test.TestHttpd)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Volumes/untitled/eventlet/tests/wsgi_test.py", line 1044, in test_unicode_raises_error
    self.assert_(response_line.startswith('HTTP/1.1 500 Internal Server Error'))
AssertionError: False is not true
```

I wasn't sure if I should put this bit of teardown code in the test itself or in ``tearDown`` method, I chose the former for the readability and because it's test-specific.