HasAttachedFile respects global settings

Normally, all of the settings would be set on `Attachment`, and they
were `deep_merge`d fine. However, if you set the global

    config.paperclip_defaults = { :validate_media_type => false }

then the `HasAttachedFile` class wouldn't pick up on it, because it only
uses things that are defined in the class. This change makes it so that
the default options are merged in to the ones that `HasAttachedFile`
uses when it sets up the attachment.

Fixes #1857

features/basic_integration.feature

@@ -12,17 +12,20 @@ Feature: Rails integration
   Scenario: Configure defaults for all attachments through Railtie
     Given I add this snippet to config/application.rb:
       """
-      config.paperclip_defaults = {:url => "/paperclip/custom/:attachment/:style/:filename"}
+      config.paperclip_defaults = {
+        :url => "/paperclip/custom/:attachment/:style/:filename",
+        :validate_media_type => false
+      }
       """
     And I attach :attachment
     And I start the rails application
     When I go to the new user page
     And I fill in "Name" with "something"
-    And I attach the file "spec/support/fixtures/5k.png" to "Attachment"
+    And I attach the file "spec/support/fixtures/animated.unknown" to "Attachment"
     And I press "Submit"
     Then I should see "Name: something"
-    And I should see an image with a path of "/paperclip/custom/attachments/original/5k.png"
-    And the file at "/paperclip/custom/attachments/original/5k.png" should be the same as "spec/support/fixtures/5k.png"
+    And I should see an image with a path of "/paperclip/custom/attachments/original/animated.unknown"
+    And the file at "/paperclip/custom/attachments/original/animated.unknown" should be the same as "spec/support/fixtures/animated.unknown"
 
   Scenario: Add custom processors
     Given I add a "test" processor in "lib/paperclip"

lib/paperclip/has_attached_file.rb

@@ -7,7 +7,7 @@ module Paperclip
     def initialize(klass, name, options)
       @klass = klass
       @name = name
-      @options = options
+      @options = Paperclip::Attachment.default_options.deep_merge(options)
     end
 
     def define