Performance regression.

### Problem
`define_attribute_methods` is slow, `AR.respond_to?` call that method, in every call:
https://github.com/rails/rails/blob/4-0-stable/activerecord/lib/active_record/attribute_methods.rb#L202

this was introduced after https://github.com/rails/rails/commit/30efdd6e9066684dc952966ed71e23a9c7640e7e .


### Solution
Make sure we dont define the methods more than one time. Almost the same way that AR does.

Benchmark using mysql+shopify Gemfile
```
Before:
       user     system      total        real
  18.800000  81.110000  99.910000 (112.324601)
After:
       user     system      total        real
   2.310000  31.300000  33.610000 ( 40.771561)
```
Benchmark using sqlite3, on vanila rails4-0 stable app
https://gist.github.com/arthurnn/b6b2dc053786b32bf94a


cc @jduff @camilo
review @byroot