bugfix: Kernel#!=, and add #!= testcase.

no need to compare the identity of the object.

* souce code(not_equal.rb)
``` ruby
str = "te\\st"
p str[2] != '\\'
```

* ruby 2.0.0
``` sh
$ ruby not_equal.rb
false
```

* current mruby
``` sh
$ ./bin/mruby not_equal.rb
true
```

* bugfixed
``` sh
$ ./bin/mruby not_equal.rb
false
```