Support "app info" for plugins in Ruby

Adds support for "app info" (a mechanism that allows a plugin's author
to identify that plugin) in Ruby. This is already supported in PHP and
we're adding it elsewhere.

README.md

@@ -122,6 +122,16 @@ an intermittent network problem:
 [Idempotency keys][idempotency-keys] are added to requests to guarantee that
 retries are safe.
 
+### Writing a Plugin
+
+If you're writing a plugin that uses the library, we'd appreciate it if you
+identified using `#set_app_info`:
+
+    Stripe.set_app_info("MyAwesomePlugin", version: "1.2.34", url: "https://myawesomeplugin.info");
+
+This information is passed along when the library makes calls to the Stripe
+API.
+
 ## Development
 
 Run all tests:

lib/stripe.rb

@@ -92,6 +92,16 @@ module Stripe
     attr_reader :max_network_retry_delay, :initial_network_retry_delay
   end
 
+  # Gets the application for a plugin that's identified some. See
+  # #set_app_info.
+  def self.app_info
+    @app_info
+  end
+
+  def self.app_info=(info)
+    @app_info = info
+  end
+
   # The location of a file containing a bundle of CA certificates. By default
   # the library will use an included bundle that can successfully validate
   # Stripe certificates.
@@ -131,6 +141,19 @@ module Stripe
     @max_network_retries = val.to_i
   end
 
+  # Sets some basic information about the running application that's sent along
+  # with API requests. Useful for plugin authors to identify their plugin when
+  # communicating with Stripe.
+  #
+  # Takes a name and optional version and plugin URL.
+  def self.set_app_info(name, version: nil, url: nil)
+    @app_info = {
+      name: name,
+      url: url,
+      version: version,
+    }
+  end
+
   private
 
   # DEPRECATED. Use `Util#encode_parameters` instead.

lib/stripe/stripe_client.rb

@@ -382,6 +382,7 @@ module Stripe
         lang_version = "#{RUBY_VERSION} p#{RUBY_PATCHLEVEL} (#{RUBY_RELEASE_DATE})"
 
         {
+          :application => Stripe.app_info,
           :bindings_version => Stripe::VERSION,
           :lang => 'ruby',
           :lang_version => lang_version,
@@ -390,7 +391,7 @@ module Stripe
           :publisher => 'stripe',
           :uname => @uname,
           :hostname => Socket.gethostname,
-        }
+        }.delete_if { |k, v| v.nil? }
       end
     end
   end

test/stripe/stripe_client_test.rb

@@ -176,6 +176,36 @@ module Stripe
         end
       end
 
+      context "app_info" do
+        should "send app_info if set" do
+          begin
+            old = Stripe.app_info
+            Stripe.set_app_info(
+              "MyAwesomePlugin",
+              url: "https://myawesomeplugin.info",
+              version: "1.2.34"
+            )
+
+            stub_request(:post, "#{Stripe.api_base}/v1/account").
+              with { |req|
+                data = JSON.parse(req.headers["X-Stripe-Client-User-Agent"],
+                  symbolize_names: true)
+
+                data["application"] == {
+                  name: "MyAwesomePlugin",
+                  url: "https://myawesomeplugin.info",
+                  version: "1.2.34"
+                }
+              }.to_return(body: JSON.generate(API_FIXTURES.fetch(:account)))
+
+            client = StripeClient.new
+            client.execute_request(:post, '/v1/account')
+          ensure
+            Stripe.app_info = old
+          end
+        end
+      end
+
       context "error handling" do
         should "handle error response with empty body" do
           stub_request(:post, "#{Stripe.api_base}/v1/charges").

test/stripe_test.rb

@@ -14,6 +14,24 @@ class StripeTest < Test::Unit::TestCase
     end
   end
 
+  should "allow app_info to be configured" do
+    begin
+      old = Stripe.app_info
+      Stripe.set_app_info(
+        "MyAwesomePlugin",
+        url: "https://myawesomeplugin.info",
+        version: "1.2.34"
+      )
+      assert_equal({
+        name: "MyAwesomePlugin",
+        url: "https://myawesomeplugin.info",
+        version: "1.2.34"
+      }, Stripe.app_info)
+    ensure
+      Stripe.app_info = old
+    end
+  end
+
   should "allow ca_bundle_path to be configured" do
     begin
       old = Stripe.ca_bundle_path