Fix arity of Hash#to_proc [Bug #12671]

hash.c

@@ -5285,7 +5285,7 @@ hash_proc_call(RB_BLOCK_CALL_FUNC_ARGLIST(key, hash))
 static VALUE
 rb_hash_to_proc(VALUE hash)
 {
-    return rb_func_proc_new(hash_proc_call, hash);
+    return rb_func_lambda_new(hash_proc_call, hash, 1, 1);
 }
 
 static VALUE

spec/ruby/core/hash/to_proc_spec.rb

@@ -29,6 +29,10 @@ describe "Hash#to_proc" do
       it "is a lambda" do
         @proc.should.lambda?
       end
+
+      it "has an arity of 1" do
+        @proc.arity.should == 1
+      end
     end
 
     it "raises ArgumentError if not passed exactly one argument" do