define Object#blank? only if not defined yet

Similar to #353

I found comment here is telling a lie https://github.com/mikel/mail/blob/9c16d9d/lib/mail/core_extensions/object.rb#L3

```
% ruby -we "require 'active_support/all'; require 'mail'"
.../gems/mail-2.5.3/lib/mail/core_extensions/object.rb:6: warning: method redefined; discarding old blank?
.../gems/activesupport-3.2.9/lib/active_support/core_ext/object/blank.rb:15: warning: previous definition of blank? was here
```

Either way, I think it's better to let Ruby check the existence of the target method before monkey-patching.