Fix partition spec and transforms in python

[jun-he]

This PR fixed a few bugs/issues in partition spec and transforms classes

• In partition spec source_column.types should be source_column.type
• type_id is missed or is mistakenly used as type_id() in multiple classes
• In dates class, __str__ will cause RecursionError as it calls itself.
• Fixed returning TruncateLong instead of TruncateInteger for TypeID.LONG.
• Fixed truncate static method to use the right input parameter type_var.
• added unit tests in python, similar to Java partition spec base test.

[rdblue]

Thanks, @jun-he! Looks good to me.