Fixes a bug where the default attribute maps were prioritised over
user-specified ones, resulting in incorrect mappings.

The problem is that if you call SPConfig.load() multiple times, it adds new
attribute mappers to a list. So by calling it with the default config first,
and then the user-specified config, we would always get the default mappers
before the user-specified mappers.

To solve this, let's merge the config dicts first, and then pass them to
SPConfig.