Unnecessary base64 decoding

[kmike]

It seems the library decodes httpResponseBody from base64 multiple times:

• First, when detecting the response type:

  scrapy-zyte-api/scrapy_zyte_api/responses.py

  Line 197 in 1269ebb

  body=b64decode(api_response["httpResponseBody"]), # type: ignore

• And then when the response is created:

  scrapy-zyte-api/scrapy_zyte_api/responses.py

  Line 127 in 1269ebb

  body = b64decode(api_response["httpResponseBody"])

  or

  scrapy-zyte-api/scrapy_zyte_api/responses.py

  Line 154 in 1269ebb

  body=b64decode(api_response.get("httpResponseBody") or ""),

This looks like a potentially costly operation; it'd be great to optimize it, so that the decoding only happens once.