Fix serialization of attribute whose name matches the serializer prefix underscored

Removed unneeded method aliasing of the prefix of the serializer name to the attr_reader :object, I don't see any usage of it other than in test_fake.rb which I fixed to reference attr_reader :object itself.
The method alias is causing a problem with serialization of an accessor method whose name either mathces the prefix of the serializer of the underscored prefix of it.

Here's an example:

``` ruby
class SomeSerializer < ActiveModel::Serializer
  attributes :some
end

class SomeObject < Struct.new(:some)
end
```

Current behavior:
``` ruby
object = SomeObject.new("some value")
SomeSerializer.new(object, {}).as_json
=> {:some=>{:some=>#<struct SomeObject some="some value">}}
```

Here is what is suppose to happen:
``` ruby
object = SomeObject.new("some value")
SomeSerializer.new(object, {}).as_json
=> {:some=>{:some=>"some value"}}
```