fixed invalid content-length bug

SourceCode
-----------------
```
    def test_if_html_data_gt_max_length(self):¶
        self.register_url_with_forcing_headers(forcing_headers={¶
            'content-length': '100a',¶
        }, body='x' * (config.MAX_CONTENT_LENGTH + 1))
```

Error Traceback 
----------------------
```
======================================================================
ERROR: test_if_html_data_gt_max_length (kscomm.spider.tests.test_fetch.HttpContentLengthTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/httpretty/core.py", line 804, in wrapper
    return test(*args, **kw)
  File "/var/data/users/chenj/myhome/gitlab404_projects/kscomm_dev/kscomm/spider/tests/test_fetch.py", line 371, in test_if_html_data_gt_max_length
    }, body='x' * (config.MAX_CONTENT_LENGTH + 1))
  File "/var/data/users/chenj/myhome/gitlab404_projects/kscomm_dev/kscomm/spider/tests/test_fetch.py", line 202, in register_url_with_forcing_headers
    forcing_headers=forcing_headers
  File "/usr/local/lib/python2.6/dist-packages/httpretty/core.py", line 684, in register_uri
    cls.Response(method=method, uri=uri, **headers),
  File "/usr/local/lib/python2.6/dist-packages/httpretty/core.py", line 706, in Response
    return Entry(method, uri, **headers)
  File "/usr/local/lib/python2.6/dist-packages/httpretty/core.py", line 380, in __init__
    self.validate()
  File "/usr/local/lib/python2.6/dist-packages/httpretty/core.py", line 399, in validate
    if igot > self.body_length:
UnboundLocalError: local variable 'igot' referenced before assignment
```