Set the request.Body to http.NoBody if request.Body is nil

recorder: specifying a cassette name is required

Also remove the `WithCassette` option.

CHANGELOG: document v4.0.0

Retract v3.2.1

Default Matcher with version v3.2.1 is stricter, so retract the version.

add NewMatcherFunc

Update CHANGELOG

Update changelog and add entry for v3.1.2

Update CHANGELOG for v3.1.1

Additional tests for invalid recorder mode and ModeRecordOnly

Add IsRecording() and IsNewCassette() methods