Fixes tests for Stripe 5.19.0

As of 5.19.0, the compute_signature method was made public and they
split the timestamp into a separate param (which must be a Time). This
version checks compute_signature based on the number of params it takes
since doing a version check felt odd. Now that it is public, the method
shouldn't change it's API.

1: stripe/stripe-ruby#915

spec/controllers/webhook_controller_spec.rb

@@ -12,10 +12,19 @@ def stub_event(identifier)
 
   def generate_signature(params, secret)
     payload   = params.to_json
-    timestamp = Time.now.to_i
-    signature = Stripe::Webhook::Signature.send(:compute_signature, "#{timestamp}.#{payload}", secret)
-
-    "t=#{timestamp},v1=#{signature}"
+    timestamp = Time.now
+
+    # compute_signature was private until version 5.19.0 when it was made
+    # public and had it's API changed to split timestamp to a separate field.
+    signer = Stripe::Webhook::Signature.method(:compute_signature)
+    signature =
+      if signer.arity == 3
+        signer.call(timestamp, payload, secret)
+      else
+        signer.call("#{timestamp.to_i}.#{payload}", secret)
+      end
+
+    "t=#{timestamp.to_i},v1=#{signature}"
   end
 
   def webhook(signature, params)