Change _initialize() to set the object name only if none predefined.  Fixes #4733

This follows the same pattern as object plural and table name, and gives devs the freedom to set their own name.  This is useful, for example, if you want to translate "Model_FooBar" to "foo_bar" instead of "foobar".

Bug report at:
http://dev.kohanaframework.org/issues/4733