Update tests for Add, Mul and Pow

Currently there are no specs for Add, Mul and Pow. The operators are tested in basic_spec.rb but only checked if they are a Basic. New spec files should be created (eg. add_spec.rb, mul_spec.rb and pow_spec.rb) that check the result of binary operations specifically to be an instance of Add, Mul or Pow and the tests also need to be moved out of basic_spec.rb

The source was covered in #17 .