bugfix: Kernel#!=, and add #!= testcase.

src/kernel.c

@@ -108,7 +108,7 @@ mrb_obj_not_equal_m(mrb_state *mrb, mrb_value self)
   mrb_bool eql_p;
 
   mrb_get_args(mrb, "o", &arg);
-  eql_p = mrb_obj_equal(mrb, self, arg);
+  eql_p = mrb_equal(mrb, self, arg);
 
   return mrb_bool_value(!eql_p);
 }

test/t/kernel.rb

@@ -349,3 +349,13 @@ end
 assert('Kernel#to_s', '15.3.1.3.46') do
   to_s.class == String
 end
+
+assert('Kernel#!=') do
+  str1 = "hello"
+  str2 = str1
+  str3 = "world"
+
+  (str1[1] != 'e') == false and
+  (str1 != str3) == true and
+  (str2 != str1) == false
+end