Generalized workaround for JDBC 4.1 getObject(index, requiredType) failures on MySQL and Derby

Issue: SPR-12174
Issue: SPR-12157
Accept empty Content-Type header

Issue: SPR-12173
CompositePropertySource allows for access to its nested property sources

Issue: SPR-12182
GenericConversionService detects enum subclasses as well

Issue: SPR-12181
CachedIntrospectionResults uses ConcurrentReferenceHashMap for its type descriptor cache

Issue: SPR-12185
Polishing