Accept: */* header does not pass mustAccept

[avallee]

Using the handler mustAccept ["text/html"] with a request with the exact accept header: "/" fails by skipping the pipeline.

According to rfc7231 Section 5.3.2

The asterisk "" character is used to group media types into ranges,
with "/" indicating all media types and "type/" indicating all
subtypes of that type. The media-range can include media type
parameters that are applicable to that range.

[Banashek]

This does look to be broken.
Fortunately, we can do the Giraffe thing and lean on the MediaTypeHeaderValue class which has a isSubsetOf method that should help in this scenario.

I've created a PR with tests I'll add.