Added content-type check as per issue #193

scrapy/contrib/downloadermiddleware/httpcompression.py

 import zlib
 
-from scrapy.utils.gz import gunzip
+from scrapy.utils.gz import gunzip, is_gzipped
 from scrapy.http import Response, TextResponse
 from scrapy.responsetypes import responsetypes
 from scrapy.exceptions import NotConfigured
@@ -22,7 +22,7 @@ class HttpCompressionMiddleware(object):
     def process_response(self, request, response, spider):
         if isinstance(response, Response):
             content_encoding = response.headers.getlist('Content-Encoding')
-            if content_encoding:
+            if content_encoding and not is_gzipped(response):
                 encoding = content_encoding.pop()
                 decoded_body = self._decode(response.body, encoding.lower())
                 respcls = responsetypes.from_args(headers=response.headers, \