Only use partially matched mappings when matching strategy is not exact.

This change excludes using partial mappings when an exact matching strategy is in use. It prevents conditional converters from performing implicit conversion from unknown types.

I am aware that using MatchingStrategy#isExact() in this way is not ideal since it will break backwards compatibility for existing strict mappers which rely on the implicit conversion from objects to primitives. Do you think it would be a good idea to add a new property to MatchingStrategy which indicates that "implicit conversion from unknown types" is allowed?