error.c: prepend "warning: " always

* error.c (rb_warn_m): prepend the string "warning: " if uplevel
  keyword is given, even if caller file and line information are
  not available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_exception.rb

@@ -1121,6 +1121,7 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
     warning = capture_warning_warn {obj.w(1)}
     assert_equal("#{__FILE__}:#{__LINE__-1}: warning: test warning\n", warning[0])
     assert_raise(ArgumentError) {warn("test warning", uplevel: -1)}
+    assert_in_out_err(["-e", "warn 'ok', uplevel: 1"], '', [], /warning:/)
   end
 
   def test_warning_warn_invalid_argument