Inconsistent Association#include?

**What happens:** Calling `include?` on Association objects sometimes returns an integer, sometimes a boolean.
**Expected:** `include?` should always return true or false.
**Occurs using:** Ruby 2.0.0p247, Rails 4.0.0

### To reproduce
1. Create association: **user has_many locations**
2. Run this:

```ruby
    user.locations.include?(location)
    => 1

    puts user.locations
    => #<Location:...>
    user.locations.include?(location)
    => true
```

`puts` is required to get the expected output with Association#include?