Allow passing options to attributes the same way as to attribute

[biomancer]

Like this

attributes :starts_at, :ends_at, format: :timestamp

[lgebhardt]

Great addition. Thanks!

[butchmarshall]

I am passing..

{..., "status-at"=>1512650542}

with JSONAPI::Resource defining

attributes :status_at, format: :timestamp

and get:

Internal Server Error: undefined method `unformat' for nil:NilClass /home/butch/.rvm/gems/ruby-2.4.2/bundler/gems/jsonapi-resources-3a02a4d2e149/lib/jsonapi/request_parser.rb:580:in `unformat_value'
/home/butch/.rvm/gems/ruby-2.4.2/bundler/gems/jsonapi-resources-3a02a4d2e149/lib/jsonapi/request_parser.rb:513:in `block (2 levels) in parse_params'
/home/butch/.rvm/gems/ruby-2.4.2/gems/actionpack-4.2.10/lib/action_controller/metal/strong_parameters.rb:185:in `each_pair'
/home/butch/.rvm/gems/ruby-2.4.2/gems/actionpack-4.2.10/lib/action_controller/metal/strong_parameters.rb:185:in `each_pair'

[senid231]

@butchmarshall it's because jsonapi-resources doesn't provide custom value formatters
you need to create your own
something like this:

class TimestampValueFormatter < JSONAPI::ValueFormatter
  class << self
    def format(raw_value)
      raw_value.to_i unless raw_value.nil?
    end

    def unformat(raw_value)
      Time.at(raw_value) unless raw_value.nil?
    end
  end
end

but I agree that error undefined method unformat'` is wrong in this case
we should reset to default value formatter and provide warning or raise correct error.
In any case I think it should be in a separate issue

@lgebhardt what do you think we should do in this case?