Add content JSON type in headers when deleting a document

With Nuxeo 5.8 : 
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When the `Content-Type` is not set to `'application/json'`, an exception is thrown by the JAX-RS 
`Exception in JAX-RS processing javax.ws.rs.WebApplicationException`

This exception disappears when the `'Content-type'` is set, and the deletion is properly made on Nuxeo platform.