Handle discriminator mapping non-ref name

Created by: kevinoid

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• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

The mapping property of the Discriminator Object is "An object to hold mappings between payload values and schema names or references." The subsequent examples in the spec have mappings of both types. The mapping support introduced in #536 only supports references.

This pull request updates the code to support names which are not references (identified by lack of /) in addition to references and changes a test mapping to cover this case.

Thanks for considering, Kevin

CC: @0v1se, @jmini (from #536) CC technical committee: @bbdouglas, @sreeshas, @jfiala, @lukoyanov, @cbornet, @jeff9finger, @karismann, @Zomzog