Fix failure message of `create_remote_file` without attributes

Ensure we return the string also if `@attributes` is nil.
Merge pull request #156 from tmatilai/create_remote_file-message

Fix failure message of `create_remote_file` without attributes
Add create_remote_file_if_missing matcher
Merge pull request #158 from jimhopp-lookout/remote-file-if-missing-matcher

Add create_remote_file_if_missing matcher