Fix error when requesting related entities in select clauses with multiple fields

[louise-davies]

In reviewing #246, @RKrahl noticed that SELECT ds.investigation, ds.name FROM Dataset ds throws an error to do with "infinitely deep XML". Upon investigation, a similar error also appears when using the REST API (500 error with logging messages indicating that jsonize caused a stackoverflow exception). This error does not occur when requesting SELECT ds.investigation FROM Dataset ds, so it is something to do with multiple field selections causing the issue.