Fixed issue #147 for POSTs to /pipelines/{id}

Issue may still exist for POSTs handled by
- com.xmlcalabash.piperack.Input
- com.xmlcalabash.piperack.Option
- com.xmlcalabash.piperack.Parameter
Fixed #147 in Input, Option, and Parameter classes
Merge pull request #151 from jwcranford/fix147

Fix #147 in Pipeline, Input, Option, and Parameter classes. Use the entity media type not the variant.