Enabling fields option to json adapter

[joaomdmoura]

It fix #992

fields option was used on Json API adapter but never on Json adapter. This implements the fields option on it.

It enables you to filter the fields you want on render method call:

class ProfileSerialization < ActiveModel::Serializer
  attributes :name, :description

  has_many :friends
end

# ProfileController

# Define the resource fields you want in the response
def show
  @profile = Profile.find(params[:id)
   render json: profile, fields: [:name]
end

# You can also define the resource relationships fields you want in the response
def show_with_some_relationship_data
  @profile = Profile.find(params[:id)
   render json: profile, fields: {profile: [:name], friends: [:id]}
end

[joaomdmoura]

Rebased with master, now with root as class name, tests are supposed to be okay! 😄

[bf4]

ref: #1058

[joaomdmoura]

It does not seems to receive a lot of back up so I'm closing it as well in favor of #1058

[vasilakisfil]

I can't believe it that this was never merged :) The API for fields was excellent.