IOError: "Not a gzipped file"

This avoids the IOError "Not a gzipped file" when the website returns
a 301/302 with 'Content-Encoding: gzip' where actually the body is not
gzip-encoded.

To reproduce:
```
$> scrapy shell http://www.laloberinto-shop.com/checkout_shipping.php
2012-01-09 14:13:11-0400 [scrapy] INFO: Scrapy 0.15.1 started (bot: bot)
...
2012-01-09 14:13:11-0400 [default] INFO: Spider opened
2012-01-09 14:13:12-0400 [scrapy] ERROR: Shell error
          ...
	  File "/opt/projects/Insophia/scrapy_git/scrapy/core/downloader/middleware.py", line 46, in process_response
	    response = method(request=request, response=response, spider=spider)
	  File "/opt/projects/Insophia/scrapy_git/scrapy/contrib/downloadermiddleware/httpcompression.py", line 20, in process_response
	    decoded_body = self._decode(response.body, encoding.lower())
	  File "/opt/projects/Insophia/scrapy_git/scrapy/contrib/downloadermiddleware/httpcompression.py", line 36, in _decode
	    body = gunzip(body)
	  File "/opt/projects/Insophia/scrapy_git/scrapy/utils/gz.py", line 14, in gunzip
	    chunk = f.read(8196)
	  File "/usr/lib/python2.7/gzip.py", line 252, in read
	    self._read(readsize)
	  File "/usr/lib/python2.7/gzip.py", line 287, in _read
	    self._read_gzip_header()
	  File "/usr/lib/python2.7/gzip.py", line 181, in _read_gzip_header
	    raise IOError, 'Not a gzipped file'
	exceptions.IOError: Not a gzipped file
```

After the fix:
```
$> scrapy shell http://www.laloberinto-shop.com/checkout_shipping.php
2012-01-09 14:15:14-0400 [scrapy] INFO: Scrapy 0.15.1 started (bot: bot)
...
2012-01-09 14:15:14-0400 [default] INFO: Spider opened
2012-01-09 14:15:14-0400 [default] DEBUG: Redirecting (302) to <GET http://www.laloberinto-shop.com/login.php> from <GET http://www.laloberinto-shop.com/checkout_shipping.php>
2012-01-09 14:15:15-0400 [default] DEBUG: Redirecting (302) to <GET http://www.laloberinto-shop.com/cookie_usage.php> from <GET http://www.laloberinto-shop.com/login.php>
2012-01-09 14:15:17-0400 [default] DEBUG: Crawled (200) <GET http://www.laloberinto-shop.com/cookie_usage.php> (referer: None)
```