Fix loading foreign key entities with base class primary key

Found the changes you made to the main sqlite-net really useful. Came across a situation with my model that needed a few tweaks, added a test to illustrate the situation "InheritanceTestWithOne2OneRelationship". Have pushed it to you if your are interested in including it in your pull request https://github.com/praeclarum/sqlite-net/pull/205 

Description:
If the parent entity had it's primary key defined in a base class the child entity would not get loaded and would return null.