Rename CheckCaseClassNulls to CaseClassCheckNulls

For clarity, as further "CaseClass" options are arriving.
Deserialize missing Option case class fields as None

The version 2.1.4 of Jacks deserializes a missing property corresponding
to an Option case class field as "null", rather than None. This leads to
situations like:

Start with ""
Deserialize -> Z(null)
Serialize -> """{"s":null}"""
Deserialize -> Z(None)

Which may be undesirable. When the option CaseClassCheckNulls is in use,
nulls should never be generated; therefore, deserialize missing properties
that correspond to Option fields as None instead, unless an explicit
default is specified.
Adding option "CaseClassSkipNulls"

Usually the annotation @JsonInclude(Include.NON_NULL) will cause null fields not to be emitted. However,
in the case of serialization of an Option[] field containing None, the standard JacksMapper will still write "null":
since the field itself is not null, even when the flag is used None is serialized as null, regardless.

This behavior cannot be changed because of compatibility reasons. Hence, this option "CaseClassSkipNulls" is introduced,
with behavior complementary to CaseClassCheckNulls. When this option is selected, each Option field that contains None will
be skipped during serialization, rather than being serialized as null. This option does not override "ALWAYS" or other
Jackson flags explicitly set on classes or individual fields: if you do, the flags will still be honored.