Use record#inspect in NotAuthorizedError message.

Hello,

I introduced pundit into an application a few months ago and most everything went well but I had a small hiccup with some ActiveRecord models that defined a `#to_s` method. Most of them returned user input for these, which ranged from benign but unhelpful printing of input instead of an object description, to bad when there was logic that failed in the method. Here is a script to demonstrate the problem:

```
# gem install rails sqlite3
gem 'rails', '~> 4'
require 'active_record'
require 'pundit'

ActiveRecord::Base.establish_connection(adapter:'sqlite3',database: ':memory:')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :tacos, :force => true do |t|
    t.string :name
    t.string :flavor
  end

  create_table :burritos, :force => true do |t|
    t.string :name
  end
end

class Taco < ActiveRecord::Base
  def to_s
    name
  end
end

class Burrito < ActiveRecord::Base
  def to_s
    name[0,10]
  end
end

class Denier
  include Pundit
  def initialize(*)
  end

  def policy(*)
    self
  end

  def method_missing(*)
    false
  end
end

def eat thing
  Denier.new.authorize thing, 'eat'
rescue => e
  puts "#{e.class}: #{e}"
end

eat Taco.new(name: 'Doritos Locos', flavor: 'Cool ranch')
eat Burrito.new
```

When I run this script on master, I get this output:

```
Pundit::NotAuthorizedError: not allowed to eat this Doritos Locos
NoMethodError: undefined method `[]' for nil:NilClass
```

When I run the script on this PR's branch, I get this output:

```
Pundit::NotAuthorizedError: not allowed to 'eat' this #<Taco id: nil, name: "Doritos Locos", flavor: "Cool ranch">
Pundit::NotAuthorizedError: not allowed to 'eat' this #<Burrito id: nil, name: nil>
```

I was curious if there was an accepted Ruby style for using `to_s` vs. `inspect`, since I wasn't sure if the problem was in pundit or this application. After searching for a few minutes, I found [this reference](http://books.google.com/books?id=jcUbTcr5XWwC&pg=PA80&lpg=PA80&dq=ruby+to_s+vs+inspect&source=bl&ots=fJApsi5rhH&sig=i37KBlQy9SP-zGpg3vyQxZ3NrrU&hl=en&sa=X&ei=dpT-U8CMO86BygSz9oKIDQ&ved=0CGQQ6AEwCA#v=onepage&q&f=false) ([imgur mirror](http://i.imgur.com/PZ4QhBb.png)) in The Ruby Programming Language by David Flanagan and Yukihiro Matsumoto:

> `to_s` is generally intended to return a human-readable representation of the object, suitable for end users. `inspect`, on the other hand, is intended for debugging use, and should return a representation that is helpful to Ruby developers.

Thanks very much for your work on this gem, I really enjoy using it!