update spec description to verify nil? on non nil object and remove duplicate spec to verify nil? for nil

Author: sampatbadhe

core/kernel/nil_spec.rb

@@ -2,15 +2,11 @@
 require_relative 'fixtures/classes'
 
 describe 'Kernel#nil?' do
-  it 'returns false for other than nil' do
+  it 'returns false' do
     Object.should_not.nil?
     Object.new.should_not.nil?
     ''.should_not.nil?
     [].should_not.nil?
     {}.should_not.nil?
   end
-
-  it 'returns true for nil' do
-    nil.should.nil?
-  end
 end