Fixed bug which was disallowing to make HEAD requests with cyclone.httpclient.

httpclient should be able to make HEAD requests now.
Script to reproduce bug fixed in my pull request:

```python
from twisted.internet import reactor
from cyclone.httpclient import fetch

resp = fetch("http://ya.ru", method="HEAD")

def ok(self, resp):
   print "OK"

def fail(self, failure):
   print "FAIL"

resp.addCallback(ok)
resp.addErrback(fail)

if __name__ == "__main__":
   reactor.run()
```

When launched with python test.py (assuming you have cyclone in your pythonpath) should hang forever.