remove 'Content-Encoding' header when returning responses

[BurnzZ]

Fixes #11. Built on top of #10.

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Lots of approach to fix it but it would seem that this is the simplest one. The only downside is that the response instance returned to Scrapy doesn't have the accurate representation of the complete headers since Content-Encoding is removed. However, that should be okay since we're preserving it anyways in response.zyte_api_response.headers.

[kmike]

The fix makes sense to me, though argument could be made that it's actually an issue with Zyte API: httpResponseBody is not a raw response, it's returned after content-encoding applied. So, our options:

1. Keep API as-is, remove header here. Pro: simple; I think that's a correct way to tell Scrapy not to decode. Cons: every client may need to do the same, if they support automatic handling of this header. Not sure if that's a big issue.
2. Keep API as-is, remove header here, documnent the API behavior propely in docs.zyte.com.
3. Remove the header in API. Pro: headers would match the content. Cons: headers won't be the same as website returns.
4. Don't decode responses in API, e.g. don't do gunzip. This is the most "pure" way, as it preserves the most information, but it could lead to some confusion on how to work with httpResponseBody, and on increased complexity for clients.

from these options, I like (2) the most. @akshayphilar any preference from you?

[akshayphilar]

@kmike agree, the second approach makes sense.