fix visibility of SecureRandom.gen_random

Aliasing a method preserves its visibility.  These aliases turn
formerly-public methods into private.  Should make them public
again.  [Bug #15847]

lib/securerandom.rb

@@ -85,6 +85,7 @@ def gen_random(n)
             class << self
               remove_method :gen_random
               alias gen_random gen_random_openssl
+              public :gen_random
             end
           end
           return gen_random(n)
@@ -94,6 +95,7 @@ class << self
           class << self
             remove_method :gen_random
             alias gen_random gen_random_urandom
+            public :gen_random
           end
         end
         return gen_random(n)

test/test_securerandom.rb

@@ -184,4 +184,11 @@ def test_with_openssl
       assert_equal(idx, @it.send(:gen_random_openssl, idx).size)
     end
   end
+
+  def test_repeated_gen_random
+    assert_nothing_raised NoMethodError, '[ruby-core:92633] [Bug #15847]' do
+      @it.gen_random(1)
+      @it.gen_random(1)
+    end
+  end
 end