[ec2] fix volume model to not muck with attributes, which breaks mocks

lib/fog/aws/models/ec2/volume.rb

@@ -22,9 +22,6 @@ module Fog
         def initialize(attributes = {})
           # assign server first to prevent race condition with new_record?
           self.server = attributes.delete(:server)
-          if attributes['attachmentSet']
-            attributes.merge!(attributes.delete('attachmentSet').first || {})
-          end
           super
         end
 
@@ -67,6 +64,10 @@ module Fog
 
         private
 
+        def attachmentSet=(new_attachment_set)
+          merge_attributes(new_attachment_set.first || {})
+        end
+
         def attach(new_server)
           if new_record?
             @server = new_server