Avoid clashing with Hashie::Mash#size and an API resource size

When asking the size of a file via API the gem returns the size of the keys representing the resource.

Example. Given an api_attachment.
```ruby
#<ZendeskAPI::Attachment {
"url"=>"http://mondocam.localhost:4000/api/v2/attachments/10121.json", 
"id"=>10121,
"file_name"=>"cat.png", 
"size"=>2698,
"content_type"=>"image/png"
}>
```
Calling api_attachment.size will return 5 instead of 2698

5 is because:

```ruby
["url", "id", "file_name", "size",  "content_type"].size = 5
```

(close HC-721)