Make the fast-path and slow-path versions of IsInitialized report
exactly the same errors: An errors.RequiredNotSet containing the
full name of one of the unset required fields.
Bugfix: Fast-path IsInitialized on a nil message reports an error only
when the message directly contains required fields.
Bugfix: Include fast-path IsInitialized in legacy messageIfaceWrapper.
Fixesgolang/protobuf#887
Change-Id: Ia5e4b386f8c23f6f855d995f4a098b1338acbae3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185397
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
