partially replicate client side tokenizeCard method

Stores credit card data identified by the returned "payment method
nonce". Returned value may be used more than once.

lib/fake_braintree.rb

@@ -12,6 +12,7 @@
 require 'fake_braintree/redirect'
 require 'fake_braintree/credit_card'
 require 'fake_braintree/address'
+require 'fake_braintree/payment_method'
 
 require 'fake_braintree/registry'
 require 'fake_braintree/server'

lib/fake_braintree/payment_method.rb

@@ -0,0 +1,9 @@
+module FakeBraintree
+  class PaymentMethod
+    def self.tokenize_card(attributes)
+      token = (Time.now.to_f * 1000).round
+      FakeBraintree.registry.payment_methods[token] = attributes
+      token
+    end
+  end
+end

lib/fake_braintree/registry.rb

@@ -4,16 +4,17 @@ def initialize
   end
 
   attr_accessor :customers,:subscriptions, :failures, :transactions, :redirects,
-    :credit_cards, :addresses
+    :credit_cards, :addresses, :payment_methods
 
   def clear!
-    @addresses     = {}
-    @customers     = {}
-    @subscriptions = {}
-    @failures      = {}
-    @transactions  = {}
-    @redirects     = {}
-    @credit_cards  = {}
+    @addresses       = {}
+    @customers       = {}
+    @subscriptions   = {}
+    @failures        = {}
+    @transactions    = {}
+    @redirects       = {}
+    @credit_cards    = {}
+    @payment_methods = {}
   end
 
   def failure?(card_number)

spec/fake_braintree/payment_method_spec.rb

@@ -33,3 +33,17 @@ def token_for(month, year)
     expect { Braintree::PaymentMethod.update('foo', number: TEST_CC_NUMBER) }.to raise_error(Braintree::NotFoundError)
   end
 end
+
+describe 'FakeBraintree::PaymentMethod.tokenize_card' do
+  it 'stores provided payment data in the registry' do
+    FakeBraintree::PaymentMethod.tokenize_card number: '4111111111111111'
+    first_payment_method = FakeBraintree.registry.payment_methods.values[0]
+    expect(first_payment_method[:number]).to eq '4111111111111111'
+  end
+
+  it 'returns key to payment data' do
+    nonce = FakeBraintree::PaymentMethod.tokenize_card number: '4111111111111111'
+    payment_methods = FakeBraintree.registry.payment_methods
+    expect(payment_methods[nonce][:number]).to eq '4111111111111111'
+  end
+end