DATAMONGO-897 - Fixed potential NullPointerException in QueryMapper.

If an association property points to an interface not containing the id property QueryMapper threw a NullPointerException in isAssociationConversionNecessary(…) as the lookup of the id property fails.

We now check for the presence of an id property on the target type and check for assignability to indicated the need for conversion (usually in case when developers use raw ids in their update clauses, not the actual target instance.

Original Pull Request: #164.
Conflicts:
	spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/UpdateMapperUnitTests.java