Merge pull request #4462 from shuujii/kernel-Rational-requires-numerator

`Kernel#Rational` requires numerator

mrbgems/mruby-rational/mrblib/rational.rb

@@ -71,7 +71,7 @@ class Numeric
 end
 
 module Kernel
-  def Rational(numerator = 0, denominator = 1)
+  def Rational(numerator, denominator = 1)
     a = numerator
     b = denominator
     a, b = b, a % b until b == 0

mrbgems/mruby-rational/test/rational.rb

@@ -27,6 +27,19 @@ assert 'Rational' do
   assert_equal([5, 1], [r.numerator, r.denominator])
 end
 
+assert 'Kernel#Rational' do
+  r = Rational(4,10)
+  assert_equal(2, r.numerator)
+  assert_equal(5, r.denominator)
+
+  r = Rational(3)
+  assert_equal(3, r.numerator)
+  assert_equal(1, r.denominator)
+
+  assert_raise(ArgumentError) { Rational() }
+  assert_raise(ArgumentError) { Rational(1,2,3) }
+end
+
 assert 'Rational#to_f' do
   assert_float(2.0, Rational(2).to_f)
   assert_float(2.25, Rational(9, 4).to_f)