Possibly munged parameter names compared against discriminators

While working on #36, I found [`discriminators.contains(param.name.value)`](https://github.com/twilio/guardrail/blob/3913d7df1155882221369712ebbd3262c9e5365a/modules/codegen/src/main/scala/com/twilio/guardrail/generators/CirceProtocolGenerator.scala#L177). This seems as though it would compare the camel-cased version of the discriminator (parameter name) with the raw field names in the discriminator list.

This should be confirmed by
1) ... writing a simple specification with a non-camel-case discriminator (like `discriminator: pet-type`)
2) ... then modify [`discriminators.contains(param.name.value)`](https://github.com/twilio/guardrail/blob/3913d7df1155882221369712ebbd3262c9e5365a/modules/codegen/src/main/scala/com/twilio/guardrail/generators/CirceProtocolGenerator.scala#L177) to compare against the original `ScalaParameter`'s `argName` (`RawParameterName`)