Remove creating subscription without a name

The Google Cloud docs state that subscriptions can be created without
a name, and that a name will be generated in such cases. However, we
cannot create subscriptions without a name using Google API Client.
Remove the option to subscribe without a name until this is supported.

[refs googleapis#193]

lib/gcloud/pubsub.rb

@@ -161,16 +161,6 @@ def self.pubsub project = nil, keyfile = nil
   #   sub = topic.subscribe "my-topic-sub"
   #   puts sub.name # => "my-topic-sub"
   #
-  # The name is optional, and will be generated if not given.
-  #
-  #   require "gcloud/pubsub"
-  #
-  #   pubsub = Gcloud.pubsub
-  #
-  #   topic = pubsub.topic "my-topic"
-  #   sub = topic.subscribe "my-topic-sub"
-  #   puts sub.name # => "generated-sub-name"
-  #
   # The subscription can be created that specifies the number of seconds to
   # wait to be acknowledged as well as an endpoint URL to push the messages to:
   #

lib/gcloud/pubsub/connection.rb

@@ -88,7 +88,7 @@ def delete_topic topic
 
       ##
       # Creates a subscription on a given topic for a given subscriber.
-      def create_subscription topic, subscription_name = nil, options = {}
+      def create_subscription topic, subscription_name, options = {}
         data = subscription_data topic, options
         @client.execute(
           api_method: @pubsub.projects.subscriptions.create,

lib/gcloud/pubsub/topic.rb

@@ -104,9 +104,11 @@ def delete
       # === Parameters
       #
       # +subscription_name+::
-      #   Name of the new subscription. If the name is not provided in the
-      #   request, the server will assign a random name for this subscription
-      #   on the same project as the topic. (+String+)
+      #   Name of the new subscription. Must start with a letter, and contain
+      #   only letters ([A-Za-z]), numbers ([0-9], dashes (-), underscores (_),
+      #   periods (.), tildes (~), plus (+) or percent signs (%). It must be
+      #   between 3 and 255 characters in length, and it must not start with
+      #   "goog". (+String+)
       # +options+::
       #   An optional Hash for controlling additional behavior. (+Hash+)
       # <code>options[:deadline]</code>::
@@ -152,7 +154,7 @@ def delete
       #                         deadline: 120,
       #                         endpoint: "https://example.com/push"
       #
-      def subscribe subscription_name = nil, options = {}
+      def subscribe subscription_name, options = {}
         ensure_connection!
         resp = connection.create_subscription name, subscription_name, options
         if resp.success?

test/gcloud/pubsub/test_topic.rb

@@ -70,19 +70,6 @@
     sub.must_be_kind_of Gcloud::Pubsub::Subscription
   end
 
-  it "creates a subscription without giving a name" do
-    mock_connection.put "/v1/projects/#{project}/subscriptions/" do |env|
-      JSON.parse(env.body)["topic"].must_equal topic_path(topic_name)
-      JSON.parse(env.body)["name"].must_be :nil?
-      [200, {"Content-Type"=>"application/json"},
-       subscription_json(topic_name, nil)]
-    end
-
-    sub = topic.subscribe
-    sub.wont_be :nil?
-    sub.must_be_kind_of Gcloud::Pubsub::Subscription
-  end
-
   it "creates a subscription with a deadline" do
     new_sub_name = "new-sub-#{Time.now.to_i}"
     deadline = 42