Ensure encoder output is a string and decoder input is a string.
Add String en/decoder.
Avoid calling enc_func twice for array encoding.

This did lead to wrong behaviour on array of ruby defined types when using quotation.
Convert and add several new test cases to SimpleType and CompositType.
Fix colmap_init. It was the wrong variable name.
Change BasicTypeMapping so that it uses type information from the DB.

Align specs to the latest type code refactoring.
Remove needless == in spec.