JSONGemEncoder.encode previously would always perform two passes. First
it would call `.as_json(options)`, but then would perform a second pass
"jsonify" to recursively call `.as_json` (this time without options)
until the data converges into a "JSON-ready" representation.

When options are not given, the second pass should be equivalent to the
first, so we can detect that, and only perform the "jsonify" step.

The only user-visible effect of this should be that we will pass no
options to `as_json` instead of an empty Hash, but implementations of
`as_json` should already be expected to accept that.