DATAMONGO-928 - Removed explicit default value for abbreviate-field-names from namespace XSD.

The default for boolean attributes leaks into the evaluation of XML namespace attributes which causes us being unable to detect whether two attributes have been set in a conflicting way.

Fix the documentation on the field-naming-strategy-ref attribute.

Original pull request: #183.
Related pull request: #177.
Related ticket: DATAMONGO-925.
DATAMONGO-925, DATAMONGO-928 - Polishing.

Only reject attribute setup if abbreviation is activated and a custom strategy is configured. Additional test cases for the rejection case and a custom, over-configuration (explicitly setting abbreviation to false, which is the default anyway).

Related pull request: #177.