Many REST servers require custom content types to be sent to them. In particular W3C Semantic compliant data stores such as Allegrograph and MarkLogic Server. Their requests will require sending RDF data (E.g. application/x-nquads) and SPARQL queries (application/sparql-query).
Currently in the Java code for the REST Client step only those content types in the Application drop down are supported. The code specifically overwrites any manually typed selection in the Application drop down. The underlying code also instructs Jersey to use only one of those mediaTypes listed, effectively ignoring the Content-Type header, if provided via the headers tab in PDI on the REST Client step.
I have written and tested a fix against my working 220.127.116.11-R installation. This can be seen in this PR: https://github.com/pentaho/pentaho-kettle/pull/5301
I have committed this change to master for the pull request, as I figured this is more helpful than committing it against the 18.104.22.168-R tag.