DATAMONGO-1049 - Check for explicitly declared language field.

We now check for an explicitly declared language field for setting language_override within a text index. Therefore the attribute (even if named with the reserved keyword language) has to be explicitly marked with @Language. Prior to this change having:

@Language String lang;
String language;

would have caused trouble when trying to resolve index structures as one cannot set language override to more than one property.

Original pull request: #224.