Extend assertion error messages

When invoked without a custom assertion message, `assert.sameValue` and
`assert.notSameValue` automatically create a message that describes the
actual and expected values.

Extend both assertion methods to also include this information in cases
where a custom message has been specified.

harness/assert.js

@@ -25,8 +25,13 @@ assert.sameValue = function (actual, expected, message) {
     }
 
     if (message === undefined) {
-        message = 'Expected SameValue(' + String(actual) + ', ' + String(expected) + ') to be true';
+        message = '';
+    } else {
+        message += ' ';
     }
+
+    message += 'Expected SameValue(' + String(actual) + ', ' + String(expected) + ') to be true';
+
     $ERROR(message);
 };
 
@@ -36,8 +41,13 @@ assert.notSameValue = function (actual, unexpected, message) {
     }
 
     if (message === undefined) {
-        message = 'Expected SameValue(' + String(actual) + ', ' + String(unexpected) + ') to be false';
+        message = '';
+    } else {
+        message += ' ';
     }
+
+    message += 'Expected SameValue(' + String(actual) + ', ' + String(unexpected) + ') to be false';
+
     $ERROR(message);
 };