`url` on a unpersisted record returns default_url

It should never be the case that `url` returns `nil`. If there is no url
that can be reasonable achieved, as is the case when the record the
attachment is attached to is unpersisted, then `url` should return the
default url.

lib/paperclip/attachment.rb

@@ -137,8 +137,6 @@ module Paperclip
     # +#for(style_name, options_hash)+
 
     def url(style_name = default_style, options = {})
-      return nil if @instance.new_record?
-
       if options == true || options == false # Backwards compatibility.
         @url_generator.for(style_name, default_options.merge(:timestamp => options))
       else

spec/paperclip/attachment_spec.rb

@@ -254,6 +254,7 @@ describe Paperclip::Attachment do
 
     context "without an Attachment" do
       before do
+        rebuild_model default_url: "default.url"
         @dummy = Dummy.new
       end
 
@@ -261,8 +262,8 @@ describe Paperclip::Attachment do
         assert !@dummy.avatar.exists?
       end
 
-      it "#url returns nil" do
-        assert_nil @dummy.avatar.url
+      it "#url returns the default_url" do
+        expect(@dummy.avatar.url).to eq "default.url"
       end
     end