pull: fix status code check in check_bintray_mirror
===================================================
Fix an issue in #2252. [Report by @ilovezfs](https://github.com/Homebrew/brew/pull/2252#issuecomment-298151694):
```
Josephs-MacBook-Pro:~ joe$ brew pull https://github.com/Homebrew/homebrew-core/pull/12969
==> Fetching patch
Patch: https://github.com/Homebrew/homebrew-core/pull/12969.patch
==> Applying patch
Applying: openssl: remove support for universal build
Warning: The Bintray mirror https://dl.bintray.com/homebrew/mirror/openssl-1.0.2k.tar.gz is not reachable (HTTP status code 200).
```
The status code of the last redirect should be 2xx to be deemed successful. I don't remember why I was checking for 3xx; most likely a typo.
DEL is 127 though, and `wget_wch` for DEL returns `OK` instead of `KEY_CODE_YES`. So, it works if you duplicate the backspace handling code in the `OK` branch for 127.