Retain behavior of Numeric#step when nil is given as second argument.

* numeric.c (NUM_STEP_SCAN_ARGS): On sencond thought, keep
  Numeral#step backward compatible in that it raises TypeError
  when nil is given as second argument.

* test/ruby/test_float.rb (TestFloat#test_num2dbl): Revert.

* test/ruby/test_numeric.rb (TestNumeric#test_step): Fix test
  cases for the above change.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42788 b2dd03c8-39d4-4d8f-98ff-823fe69b080e