Fix MacOS /tmp symlink

[litvinof]

On macOS, /tmp is by default a symbolic link to /private/tmp. (just like /var). So when nvim inializes a buffer, its name is not /tmp/response.http but rather /private/tmp/response.http.

The PR just basically makes sure that nvim still opens the set response_bufname even if it was symlinked to any other directory, which fixes the MacOS bug as well