Add new .generate_header method for webhooks (#916)

Adds a new `generate_header` method for the webhooks module, following
up #915. This method doesn't help in any way with webhook verification,
but may be useful to users in their test suites as it allows them to
easily simulate the contents of a header that Stripe might have sent.

We briefly discussed an alternative design here, but this one seems like
the best fit:
#915 (comment)

lib/stripe/webhook.rb

@@ -39,6 +39,23 @@ def self.compute_signature(timestamp, payload, secret)
                                 timestamped_payload)
       end
 
+      # Generates a value that would be added to a `Stripe-Signature` for a
+      # given webhook payload.
+      #
+      # Note that this isn't needed to verify webhooks in any way, and is
+      # mainly here for use in test cases (those that are both within this
+      # project and without).
+      def self.generate_header(timestamp, signature, scheme: EXPECTED_SCHEME)
+        raise ArgumentError, "timestamp should be an instance of Time" \
+          unless timestamp.is_a?(Time)
+        raise ArgumentError, "signature should be a string" \
+          unless signature.is_a?(String)
+        raise ArgumentError, "scheme should be a string" \
+          unless scheme.is_a?(String)
+
+        "t=#{timestamp.to_i},#{scheme}=#{signature}"
+      end
+
       # Extracts the timestamp and the signature(s) with the desired scheme
       # from the header
       def self.get_timestamp_and_signatures(header, scheme)

test/stripe/webhook_test.rb

@@ -22,7 +22,11 @@ def generate_header(opts = {})
         opts[:payload],
         opts[:secret]
       )
-      "t=#{opts[:timestamp].to_i},#{opts[:scheme]}=#{opts[:signature]}"
+      Stripe::Webhook::Signature.generate_header(
+        opts[:timestamp],
+        opts[:signature],
+        scheme: opts[:scheme]
+      )
     end
 
     context ".compute_signature" do
@@ -38,6 +42,24 @@ def generate_header(opts = {})
       end
     end
 
+    context ".generate_header" do
+      should "generate a header in valid format" do
+        timestamp = Time.now
+        signature = Stripe::Webhook::Signature.compute_signature(
+          timestamp,
+          EVENT_PAYLOAD,
+          SECRET
+        )
+        scheme = "v1"
+        header = Stripe::Webhook::Signature.generate_header(
+          timestamp,
+          signature,
+          scheme: scheme
+        )
+        assert_equal("t=#{timestamp.to_i},#{scheme}=#{signature}", header)
+      end
+    end
+
     context ".construct_event" do
       should "return an Event instance from a valid JSON payload and valid signature header" do
         header = generate_header