Don't decompress responses for HEAD requests

aiohttp causes error on receiving response against HEAD request with CONTENT-ENCODING header. 

```
Traceback (most recent call last):
  File "/home/kxepal/projects/aiocouchdb/test.py", line 94, in <module>
    loop.run_until_complete(go('http://localhost:5984'))
  File "/usr/local/opt/virtualenv/aiocouchdb/lib/python3.3/site-packages/asyncio/base_events.py", line 208, in run_until_complete
    return future.result()
  File "/usr/local/opt/virtualenv/aiocouchdb/lib/python3.3/site-packages/asyncio/futures.py", line 243, in result
    raise self._exception
  File "/usr/local/opt/virtualenv/aiocouchdb/lib/python3.3/site-packages/asyncio/tasks.py", line 319, in _step
    result = coro.send(value)
  File "/home/kxepal/projects/aiocouchdb/test.py", line 41, in go
    att = yield from doc.att('bar.txt')
  File "/home/kxepal/projects/aiocouchdb/aiocouchdb/document.py", line 58, in attachment
    yield from resp.read()
  File "/home/kxepal/projects/aiocouchdb/aiocouchdb/client.py", line 91, in read
    data.extend((yield from self.content.read()))
  File "/home/kxepal/projects/aiohttp/aiohttp/streams.py", line 382, in read
    return (yield from super().read())
  File "/home/kxepal/projects/aiohttp/aiohttp/streams.py", line 393, in read
    return (yield from super().read())
  File "/home/kxepal/projects/aiohttp/aiohttp/streams.py", line 353, in read
    raise self._exception
  File "/home/kxepal/projects/aiohttp/aiohttp/parsers.py", line 197, in set_parser
    next(p)
  File "/home/kxepal/projects/aiohttp/aiohttp/protocol.py", line 307, in __call__
    out.feed_eof()
  File "/home/kxepal/projects/aiohttp/aiohttp/protocol.py", line 386, in feed_eof
    raise errors.IncompleteRead(0)
aiohttp.errors.IncompleteRead: IncompleteRead(0 bytes read)
```

This happens because DeflateBuffer is been used if response is compressed and it tries to decompress the received data on feed_eof. Obliviously, it fails because no data is returned back on HEAD request.