Add Model.array_from_json to json_serializer, don't have Model.from_json handle arrays

The previous code made it easy for an attacker to force a single
instance when an array is desired, or an array of instance where a
single instance is desired.  This also makes it more similar
to the xml_serializer plugin.
Add spec for Sequel.parse_json
Make json_serializer from_json and array_from_json class methods secure by default

This makes both more similar to the from_json instance method by
default, using set or set_fields and not allowing the setting
of arbitrary columns or associations.  An :associations option
has been added to support specific associations, and the
:all_columns and :all_associations options have been added to get
the previous insecure behavior.

The json_create class method uses the :all_columns and
:all_associations options, so it should be fully backwards
compatible if you change Sequel.parse_json to use
:create_additions=>true when parsing JSON.

The json_create method is now considered deprecated, I will
probably be removing the :all_columns and :all_associations
options as well as the json_create method starting in Sequel 4.
Raise an error when using :all_columns with JSON containing non-columns

This restores the previous behavior.
Automatically cascade association parsing if :all_associations is used

This restores the previous behavior.
Don't have json_serializer assign to values hash directly when using :all_columns

Instead, expand the allowed setter methods to be all that are not
specifically restricted.  This makes it so that column values
are typecast correctly.
Update documentation for json_serializer to reflect reality
Make xml_serializer plugin secure by default

This contains non-backwards compatible changes to the xml_serializer
plugin, similar to the changes recently made to the json_serializer
plugin, and for the same reason.  The new API/options are very
similar to the json_serializer plugin.  You can get back the old
behavior using the :all_associations and :all_columns options.