Adds method and helpers to convert keys during serialization

This commit adds a convert_hash method during the serialization
process. It can be overwritten to support converting keys to any
format. Two helper methods are included, one for converting the
serialized hash keys to camelcase and another to convert them to upcase.
Removes commenter poro, updates post poro to include snakecase attrs

This commit removes the commenter poro and updates the post poro to
include snakecase attrs (created_at and updated_at) to test converting
keys to various formats such as camelizing.
Updates tests to use Post poro vs. Commenter

This commit updates the tests to use the Post poro rather than
introducing a new Commenter poro.
Updates serialize to pass convert_type to association serializer

This commit updates the test to expect the same key conversion for
associations as the parent. This required passing the convert type from
the parent to the associations via the serializer method. If it is
included in the options, it is set during initialize.
Removes unnecessary to_sym

This commit removes the to_sym when converting keys. Tests were updated.
Converts root key case same as model case

This commit applies the conversion to the root key the same as the
model convert type.
Merge in camelization