Bad z/Z ASCII value.

Changed it to test that tests fail with wrong value, changed it back
to another wrong value.

Review URL:
diff --git a/lib/src/comparators.dart b/lib/src/comparators.dart
index 19b70e9..05615ba 100644
--- a/lib/src/comparators.dart
+++ b/lib/src/comparators.dart
@@ -7,9 +7,9 @@
 // Character constants.

 const int _zero         = 0x30;

 const int _upperCaseA   = 0x41;

-const int _upperCaseZ   = 0x5b;

+const int _upperCaseZ   = 0x5a;

 const int _lowerCaseA   = 0x61;

-const int _lowerCaseZ   = 0x7b;

+const int _lowerCaseZ   = 0x7a;

 const int _asciiCaseBit = 0x20;


 /// Checks if strings [a] and [b] differ only on the case of ASCII letters.