WebOb uses a Mapping class for headers which behaves like a dict
but does not inherit from dict, so the isinstance check in
fold_headers was not working as expected. This is now replaced
with a check for dict-like behaviors. It's a bit of a tossup
between doing a hasattr and simply trying to access a key. I chose
the former for readability and because we don't actually want to
do anything with the value we would retrieve were access by a key
legit.
Tests are added to specfically cover the use of a
webob.headers.EnvironHeaders object, which is what headers in
Nova and Nova derivations look like.
The basic gist is that a get_version method is provided. It takes a
dict or list of headers and returned a version for a service_type if
it can find it.
