Use binary mode for the tempfile created when processing a raw response

RestClient uses a tempfile when processing a raw response. Raw responses are most-often used when downloading files; in order to prevent conversion errors, the file should be opened in binary mode.

This was first patched by by @dadiv in #110 about 2 years ago, but the patch was never merged because it was missing a test. It's since been patched by @Peeja in #145 and mentioned in various issues. I've added a test for `tempfile.binmode` in the hopes of getting it merged ;)

History:

* https://github.com/rest-client/rest-client/pull/110
* https://github.com/rest-client/rest-client/pull/145
* https://github.com/adamsb6/s3_file/issues/18