[#67] Do not decode binary response body

[robertoaloi]

Emulating the approach in #62 , this PR skips decoding of the response body for those methods where the body is a binary. Since my Elixir knowledge is quite limited, please advise on the change for the Elixir generated code (currently not included in this PR).

[robertoaloi]

I think the Elixir version requires the update of the AWS.Request module:

• https://github.com/aws-beam/aws-elixir/blob/master/lib/aws/request.ex#L44
• https://github.com/aws-beam/aws-elixir/blob/master/lib/aws/request.ex#L104

Let me know if you would like me to also check the Elixir parts or if you prefer tackling that yourself. I'm new to Elixir, so that's the slowest part for me...

[robertoaloi]

Let me know if you would like me to also check the Elixir parts or if you prefer tackling that yourself. I'm new to Elixir, so that's the slowest part for me...

Created #71 for the Elixir part, not to be blocked on the Erlang side. I will try to address it, but if you want to jump in you're welcome!