Sets the created Tempfile to binary mode during RestClient::Request#fetch_body

I was using a REST API to retrieve a large binary file, so was interested in using `raw_response` mode.

I had a character encoding exception which made it clear that attempts at translation were going on, so I set the Tempfile to binary which solved my issue.

This may not be a typical use case (perhaps people are retrieving large text-based documents that should be character-translated) but I figured I'd offer this pull request in case this seems useful.