All configuration values from `ConfigText` are potentially unicode objects and, in fact, we'd prefer them to be unicode but yaml only creates unicode strings if needed. Since HTTP method names use only ASCII characters, the unicode case never arose here.

In Vumi Go, the config is loaded from JSON and so everything ends up as unicode.

Possibly we should rename `ConfigText`, `ConfigUnicode` and always coerce the YAML results to unicode there, but I think that's a separate can of worms (and won't change what happens in this PR anyway).