Test unmapped reads functionality

[macieksmuga]

Write a test to verify that a reads/search query with no referenceId specified returns all reads including umapped reads, per the schemas:

  /**
  The reference to query. Leaving blank returns results from all
  references, including unmapped reads - this could be very large.
  */
  union { null, string } referenceId = null;

(schemas/blob/master/src/main/resources/avro/readmethods.avdl lines 22-26)

[david4096]

The issue on server: ga4gh/ga4gh-server#287