Allow factories to use RSpec example methods such as `double`

This can either be an improvement or something to add to the documentation as suggested by Daniel J Colson on Twitter.

The simple feature is to, at least, use RSpec's double within a factory's initialize_with.

How

RSpec.configure do |config|
  config.include FactoryBot::Syntax::Methods

  config.before(:suite) do
    FactoryBot.find_definitions
    FactoryBot::Evaluator.include RSpec::Mocks::ExampleMethods
  end
end

Why?

I've written a test suite for a third party API and I need to mock the response. For external requests I'm using Webmock with VCR but that has no bearing on this particular issue. The issue is to test my own code before any external API ever needs to be hit. Here's a sample:

FactoryBot.define do
  Response = Trav3::Response
  factory :response do
    initialize_with do
      dbl = double
      allow(dbl).to receive(:body).and_return( { a: :b, c: :d }.to_json )
      new(build(:travis), dbl)
    end
  end
end

Now I can use the nice and readable format of double within initialize_with and don't have to resort to:

dbl = Object.new
dbl.define_singleton_method(:body) { { a: :b, c: :d }.to_json }

Start by reproducing the initialize_with example using FactoryBot::Evaluator and RSpec::Mocks::ExampleMethods, then inspect how FactoryBot evaluates initialize_with blocks. Compare the requested double and allow calls with the documented configuration workaround. Done means the example works through the supported factory setup, with coverage or documentation reflecting the chosen behavior.