[utils] Remove Content-encoding from headers after decompression

With cn_verification_proxy, our http_response() is called twice, one from
PerRequestProxyHandler.proxy_open() and another from normal
YoutubeDL.urlopen(). As a result, for proxies honoring Accept-Encoding, the
following bug occurs:

$ youtube-dl -vs --cn-verification-proxy https://secure.uku.im:993 "test:letv"
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['-vs', '--cn-verification-proxy', 'https://secure.uku.im:993', 'test:letv']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.12.23
[debug] Git HEAD: 97f18fa
[debug] Python version 3.5.1 - Linux-4.3.3-1-ARCH-x86_64-with-arch-Arch-Linux
[debug] exe versions: ffmpeg 2.8.4, ffprobe 2.8.4, rtmpdump 2.4
[debug] Proxy map: {}
[TestURL] Test URL: http://www.letv.com/ptv/vplay/22005890.html
[Letv] 22005890: Downloading webpage
[Letv] 22005890: Downloading playJson data
ERROR: Unable to download JSON metadata: Not a gzipped file (b'{"') (caused by OSError('Not a gzipped file (b\'{"\')',)); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
  File "/home/yen/Executables/Multimedia/youtube-dl/youtube_dl/extractor/common.py", line 330, in _request_webpage
    return self._downloader.urlopen(url_or_request)
  File "/home/yen/Executables/Multimedia/youtube-dl/youtube_dl/YoutubeDL.py", line 1886, in urlopen
    return self._opener.open(req, timeout=self._socket_timeout)
  File "/usr/lib/python3.5/urllib/request.py", line 471, in open
    response = meth(req, response)
  File "/home/yen/Executables/Multimedia/youtube-dl/youtube_dl/utils.py", line 773, in http_response
    raise original_ioerror
  File "/home/yen/Executables/Multimedia/youtube-dl/youtube_dl/utils.py", line 761, in http_response
    uncompressed = io.BytesIO(gz.read())
  File "/usr/lib/python3.5/gzip.py", line 274, in read
    return self._buffer.read(size)
  File "/usr/lib/python3.5/gzip.py", line 461, in read
    if not self._read_gzip_header():
  File "/usr/lib/python3.5/gzip.py", line 409, in _read_gzip_header
    raise OSError('Not a gzipped file (%r)' % magic)

youtube_dl/utils.py

@@ -773,11 +773,13 @@ class YoutubeDLHandler(compat_urllib_request.HTTPHandler):
                     raise original_ioerror
             resp = self.addinfourl_wrapper(uncompressed, old_resp.headers, old_resp.url, old_resp.code)
             resp.msg = old_resp.msg
+            del resp.headers['Content-encoding']
         # deflate
         if resp.headers.get('Content-encoding', '') == 'deflate':
             gz = io.BytesIO(self.deflate(resp.read()))
             resp = self.addinfourl_wrapper(gz, old_resp.headers, old_resp.url, old_resp.code)
             resp.msg = old_resp.msg
+            del resp.headers['Content-encoding']
         # Percent-encode redirect URL of Location HTTP header to satisfy RFC 3986 (see
         # https://github.com/rg3/youtube-dl/issues/6457).
         if 300 <= resp.code < 400: