Fix compiler error message for optional case class containing custom type.

[andrewsmartin]

Example:

case class Record(x: String, y: Option[Inner])
case class Inner(f: URI)
ExampleConverter[Record]

[error] /Users/amartin/repos/tfexample-derive/core/src/test/scala/com/spotify/tfexample/derive/ExampleConverterTest.scala:197:21: magnolia.Deferred is used for derivation of recursive typeclasses
[error]     ExampleConverter[Record]
[error]                     ^
[error] one error found
[error] (core / Test / compileIncremental) Compilation failed

[andrewsmartin]

Related: softwaremill/magnolia#154

[nevillelyh]

This is no longer an issue with magnolify right? Since we're covering nested records here:
https://github.com/spotify/magnolify/blob/master/tensorflow/src/test/scala/magnolify/tensorflow/test/ExampleTypeSpec.scala#L90