Changed AnnotationContentValuesMapper to delegate to a NamingStrategy
when determining the name under which to store a field's value when a
name isn't set on its ContentValue annotation.  The strategy can be set
using
AnnotationContentValuesMapper#setKeyNamingStrategy(NamingStrategy(NamingStrategy).

Two NamingStrategy implementations currently exist
DefaultNamingStrategy, which returns the given field name as-is, and
UnderscoredNamingStrategy, which converts camelCase names to
lowercase_with_underscores.