Add spec and untag test for Rational.new.

Author: aardvark179

spec/ruby/core/rational/rational_spec.rb

@@ -4,4 +4,8 @@
   it "includes Comparable" do
     Rational.include?(Comparable).should == true
   end
+
+  it "does not respond to new" do
+    -> { Rational.new(1) }.should raise_error(NoMethodError)
+  end
 end

test/mri/excludes/Rational_Test.rb

@@ -5,7 +5,6 @@
 exclude :test_marshal_compatibility, "needs investigation"
 exclude :test_parse, "needs investigation"
 exclude :test_power_of_0, "needs investigation"
-exclude :test_ruby19, "needs investigation"
 exclude :test_supp, "needs investigation"
 exclude :test_to_f, "needs investigation"
 exclude :test_Rational_without_exception, "needs investigation"