allow_nil for strings and inverse logic of allow_nil

[morr]

1. I added allow_nil option to String type so it works similar to numeric types. Because there is no reason why you should forbid nil values in string fields.

2. Inverted logic of allow_nil because when you have such class

   class A
   include ShallowAttributes
   attribute :a, Integer, allow_nil: true
   end

and you initializing a with nil

A.new(a: nil)

you expect it's value to be nil, cause you specifically wrote allow_nil: true in class definition. But currently it has 0 value.

pry(main)> A.new(a: nil).a
0

[coveralls]

Coverage increased (+0.0007%) to 99.879% when pulling 2885c3ab6ca1063b9aabea7942f912b981608206 on morr:nil-strings into 37fe19415397c78fcc40c562ea6b0b0b7c47cb16 on davydovanton:master.