* fix comments
* don't export default ReflectedEncoder fn
* move newReflectEncoder nil check to NewJsonEncoder()

The defaultReflectedEncoder function can satisfy the signature for
`NewReflectedEncoder`, so we don't need to return a closure.

And the returned `json.Encoder` matches the ReflectedEncoder interface
so we don't need the wrapper type either.

assertOutput manually sets up a jsonEncoder.
This means that any config initalization we do in the constructor has to
be replicated in assertOutput.
Instead, use the constructor and cast to `*jsonEncoder`.

Co-authored-by: Sung Yoon Whang <sungyoonwhang@gmail.com>