Add support for creating a PaymentIntent via the sample backend (#34)

* Making all parameters controllable by client
* Pass back intent ID + secret
* Update Stripe ruby library to 3.17 for PaymentIntents. Support was added in 3.16: https://github.com/stripe/stripe-ruby/pull/657/files

Gemfile

@@ -6,4 +6,4 @@ gem 'dotenv', '2.2.1'
 gem 'encrypted_cookie', '0.0.5'
 gem 'json', '2.1.0'
 gem 'sinatra', '2.0.2'
-gem 'stripe', '3.11.0'
+gem 'stripe', '3.17.0'

Gemfile.lock

@@ -17,7 +17,7 @@ GEM
       rack (~> 2.0)
       rack-protection (= 2.0.2)
       tilt (~> 2.0)
-    stripe (3.11.0)
+    stripe (3.17.0)
       faraday (~> 0.10)
     tilt (2.0.8)
 
@@ -29,7 +29,7 @@ DEPENDENCIES
   encrypted_cookie (= 0.0.5)
   json (= 2.1.0)
   sinatra (= 2.0.2)
-  stripe (= 3.11.0)
+  stripe (= 3.17.0)
 
 RUBY VERSION
    ruby 2.5.0p0

web.rb

@@ -105,6 +105,29 @@ def authenticate!
   return log_info("Charge successfully created")
 end
 
+# This endpoint is used by the mobile example apps to create a PaymentIntent.
+# https://stripe.com/docs/api#create_payment_intent
+# Just like the `/create_charge` endpoint, a real implementation would include controls
+# to prevent misuse
+post '/create_intent' do
+  begin
+    intent = Stripe::PaymentIntent.create(
+      :allowed_source_types => ['card'],
+      :amount => params[:amount],
+      :currency => params[:currency] || 'usd',
+      :description => params[:description] || 'Example PaymentIntent charge',
+      :return_url => params[:return_url],
+    )
+  rescue Stripe::StripeError => e
+    status 402
+    return log_info("Error creating payment intent: #{e.message}")
+  end
+
+  log_info("Payment Intent successfully created")
+  status 200
+  return {:intent => intent.id, :secret => intent.client_secret}.to_json
+end
+
 # This endpoint responds to webhooks sent by Stripe. To use it, you'll need
 # to add its URL (https://{your-app-name}.herokuapp.com/stripe-webhook)
 # in the webhook settings section of the Dashboard.