fix HTTP-violating excess whitespace in write_error output

Excess whitespace in `write_error` output yields excess data that doesn't match `Content-Length`. I found this after finding *'upstream sent more data than specified in "Content-Length" header while reading upstream'* messages in nginx error log (matching "internal server error" responses). It also can be obseved with curl:
```
#test.py
def app(environ, start_response):
  raise Exception('test')
```
```
gunicorn --preload --error-logfile - test:app
```
```
$ curl -v http://localhost:8000/ 2>&1 | grep -i excess
* Excess found in a non pipelined read: excess = 1, size = 141, maxdownload = 141, bytecount = 0
```

The fix is really trivial and straightforward, should i rearrange it as an issue-fix-branch?
Also if a unit test is needed, I need some guidance regarding usage of current testing system.
I ran `tox` and existing tests passed for py27.