Fixed an error when "Content-Length" is not int like

When I write some unittest in HTTPretty, need test some special "HTTP Headers" (like Content-Lenght is not int), but is runs error.

like that(I changed some HTTPretty's unittest code):
```shell
Traceback (most recent call last):
  File "/home/kun/projects/HTTPretty/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/kun/projects/HTTPretty/local/lib/python2.7/site-packages/nose/util.py", line 613, in newfunc
    return func(*arg, **kw)
  File "/home/kun/projects/HTTPretty/httpretty/core.py", line 1013, in wrapper
    return test(*args, **kw)
  File "/home/kun/projects/HTTPretty/local/lib/python2.7/site-packages/sure/__init__.py", line 186, in wrap
    func(*args, **kw)
  File "/home/kun/projects/HTTPretty/tests/functional/test_urllib2.py", line 136, in test_httpretty_should_allow_forcing_headers_urllib2
    'Content-Length': '35a',
  File "/home/kun/projects/HTTPretty/httpretty/core.py", line 892, in register_uri
    cls.Response(method=method, uri=uri, **headers),
  File "/home/kun/projects/HTTPretty/httpretty/core.py", line 915, in Response
    return Entry(method, uri, **headers)
  File "/home/kun/projects/HTTPretty/httpretty/core.py", line 502, in __init__
    self.validate()
  File "/home/kun/projects/HTTPretty/httpretty/core.py", line 521, in validate
    if igot > self.body_length:
UnboundLocalError: local variable 'igot' referenced before assignment
```

So, I edit some core.py's code and change the HTTPretty's unittest, for include this damn situation.