GORA-207: Fixing some bugs. This is the last patch about GORA-207 (hopefully).

Full support of unions (2 types unions, 3 types unions,...) :

- Support of ["null",type] (a.k.a. optional field).
- Support for mutitypes(3+) unions.
- Support of nested unions.
- Support of recursive optional records.
- Support of unions as value in maps and arrays.
- Serialization of topmost optional fields of the main record in "raw": topmost ["null","type"] (optional field) will be persisted like if it was ["type"] (and non-existant column === null). This ensures data form 0.2.1 can be read.

git-svn-id: https://svn.apache.org/repos/asf/gora/branches/GORA_174@1480130 13f79535-47bb-0310-9956-ffa450edef68