Make Test adapter constructor able to receive stubs as a keyword argument

[alanoliveira]

Description

This PR makes Test adapter constructor able to receive stubs as a keyword argument.

Details

The Test adapter expects to receive the stubs as a positional argument. It is great because in the most of cases we want to use it like

Faraday.new do |builder|
  builder.adapter :test, stubs
end

But it is not compatible with Faraday.default_adapter_options as it must to be a hash.

faraday/lib/faraday/rack_builder.rb

Lines 72 to 76 in 59c5003

	def build
	raise_if_locked
	block_given? ? yield(self) : request(:url_encoded)
	adapter(Faraday.default_adapter, **Faraday.default_adapter_options) unless @adapter
	end

So we can't do something like

RSpec.configure do |config|
  config.before :context, :stub_api do
    Faraday.default_adapter = :test
    Faraday.default_adapter_options = # must be a hash
  end
end

I changed the Test adapter constructor to also accept positional argument instead of change RackBuilder::Handler#builder method because the way to pass options to adapters is using hash.