fix to issue 1293 - Include :: data corruption when doing query with include on One-to-One navigation property

For One to One PK-PK relationship we were creating INNER JOINs, which could result in entities not being returned for Include query if their referenced entity was null. Fix is to produce LEFT JOIN instead for One to One Include queries.
@ajcvickers 
Alternatively, for PK-PK Foreign Key we could set IsRequired to false? 
This way we would INNER JOINS for cases where they are safe (PK-FK or PK-shadowFK). Would that have some side-effects?