Do not require charset=utf-8 for JSON

[koterpillar]

Per JSON Interchange format, charset isn't needed for JSON as it can be assumed to be in UTF-8.

The JSON response matcher thus doesn't need to enforce charset=utf-8 even when the response contains non-ASCII characters.

Fixes #39.

[sol]

Sorry for the late reply, and thank you for bringing this up.

Apparently specifying a charset is not only redundant, but even illegal according to the content type registration.

So what do we actually want to do here? I think long-term we want to reject everything about application/json. As hspec does not provide a mechanism to issue warnings, we may do that change even now, without giving users a grace period.

Users that want to test nonstandard behavior are still free to specify their own expected headers.

[sol]

http://www.iana.org/assignments/media-types/application/json

[koterpillar]

That makes everything even easier. The code now only accepts application/json and doesn't bring in new dependencies.

[sol]

I went for an even simpler solution using <:>.