`module ActiveStorage`, not `ActiveStorage::Class`

The reasons for this commit are:

- uniformity with the other Rails libraries
- (possibly) behave better with respect to autoloading
- fix the index in the generated documentation

Before this commit, run `rake rdoc` generates this left sidebar
(ActiveStorage entries are indexed twice, both inside and outside the module):

<img width="308" alt="before" src="https://user-images.githubusercontent.com/10076/28939523-7c087dec-7846-11e7-9289-38ed4a2930cd.png">

After this commit, run `rake rdoc` generates this left sidebar:
(ActiveStorage entries are only indexed inside the module):

<img width="303" alt="after" src="https://user-images.githubusercontent.com/10076/28939524-7c090be0-7846-11e7-8ee5-29dfecae548e.png">

1 files changed, 24 insertions, 22 deletions

diff --git a/activestorage/lib/active_storage/attached.rb b/activestorage/lib/active_storage/attached.rb
index 2dbf841864..5ac8ba5377 100644
--- a/activestorage/lib/active_storage/attached.rb
+++ b/activestorage/lib/active_storage/attached.rb
@@ -2,33 +2,35 @@ require "action_dispatch"
 require "action_dispatch/http/upload"
 require "active_support/core_ext/module/delegation"
 
+module ActiveStorage
 # Abstract baseclass for the concrete `ActiveStorage::Attached::One` and `ActiveStorage::Attached::Many`
 # classes that both provide proxy access to the blob association for a record.
-class ActiveStorage::Attached
-  attr_reader :name, :record
+  class Attached
+    attr_reader :name, :record
 
-  def initialize(name, record)
-    @name, @record = name, record
-  end
+    def initialize(name, record)
+      @name, @record = name, record
+    end
 
-  private
-    def create_blob_from(attachable)
-      case attachable
-      when ActiveStorage::Blob
-        attachable
-      when ActionDispatch::Http::UploadedFile
-        ActiveStorage::Blob.create_after_upload! \
-          io: attachable.open,
-          filename: attachable.original_filename,
-          content_type: attachable.content_type
-      when Hash
-        ActiveStorage::Blob.create_after_upload!(attachable)
-      when String
-        ActiveStorage::Blob.find_signed(attachable)
-      else
-        nil
+    private
+      def create_blob_from(attachable)
+        case attachable
+        when ActiveStorage::Blob
+          attachable
+        when ActionDispatch::Http::UploadedFile
+          ActiveStorage::Blob.create_after_upload! \
+            io: attachable.open,
+            filename: attachable.original_filename,
+            content_type: attachable.content_type
+        when Hash
+          ActiveStorage::Blob.create_after_upload!(attachable)
+        when String
+          ActiveStorage::Blob.find_signed(attachable)
+        else
+          nil
+        end
       end
-    end
+  end
 end
 
 require "active_storage/attached/one"