avoid infinite regress in __getattr__

After a large amount of debugging a deadlock in pip's test suite, I found an infinite recursion in the below `__getattr__` method. I've fixed it and regression-tested it as best I could.

It's not clear to me how an attribute of this class is referenced after its __fp is deleted, but it did happen.

For historicity's sake, this is the stack that caused the inifinite recursion:


```python
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/workspace/venv/bin/pip", line 9, in <module>
    load_entry_point('pip==1.6.dev1', 'console_scripts', 'pip')()
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/__init__.py", line 198, in main
    return command.main(cmd_args)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/basecommand.py", line 212, in main
    status = self.run(options, args)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/commands/install.py", line 317, in run
    requirement_set.prepare_files(finder)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/req/req_set.py", line 238, in prepare_files
    req_to_install, self.upgrade)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/index.py", line 284, in find_requirement
    for page in self._get_pages(url_locations, req):
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/index.py", line 391, in _get_pages
    page = self._get_page(location, req)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/index.py", line 611, in _get_page
    result = HTMLPage.get_page(link, req, session=self.session)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/index.py", line 693, in get_page
    "Cache-Control": "max-age=600",
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/sessions.py", line 463, in get
    return self.request('GET', url, **kwargs)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/download.py", line 286, in request
    result = super(PipSession, self).request(method, url, *args, **kwargs)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/sessions.py", line 451, in request
    resp = self.send(prep, **send_kwargs)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/sessions.py", line 557, in send
    r = adapter.send(request, **kwargs)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/cachecontrol/adapter.py", line 38, in send
    return self.build_response(request, cached_response, from_cache=True)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/cachecontrol/adapter.py", line 95, in build_response
    request, response
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/adapters.py", line 203, in build_response
    response.headers = CaseInsensitiveDict(getattr(resp, 'headers', {}))
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/structures.py", line 46, in __init__
    self.update(data, **kwargs)
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/workspace/venv/lib/python2.7/_abcoll.py", line 541, in update
    for key in other:
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/requests/packages/urllib3/response.py", line 304, in closed
    elif hasattr(self._fp, 'isclosed'):  # Python 2
  File "/tmp/pytest-29/test_upgrade_user_conflict_in_globalsite0/pip_src/pip/_vendor/cachecontrol/filewrapper.py", line 27, in __getattr__
    f.write(''.join(traceback.format_stack()))
```

I believe that gc is activating during Mapping.update, and jumping to response.closed. Why it would touch a property, I don't know. Further, this is happening after something (the gc I assume) has deleted `__fp` from the filewrapper. Because filewrapper references `self.__fp` in `__getattr__`, and `__fp` isn't present, it recurses there. Why I don't get a StackOverflowError rather than a busy deadlock, I also don't know.

But, this fixes it :D
As said in IRC, I think checking explicitly for the attribute name in the ``__getattr__`` is cleaner, although if this style is kept, does ``self.__getattribute__("_CallbackFileWrapper__fp")`` not work?