New: CPString 'compare:' method handles nil and CPNull in correct way

This is the same way as Cocoa works.
Also added test case.

Foundation/CPString.j

@@ -27,6 +27,7 @@
 @import "CPSortDescriptor.j"
 @import "CPURL.j"
 @import "CPValue.j"
+@import "CPNull.j"
 
 @class CPException
 @class CPURL
@@ -496,6 +497,9 @@ var CPStringUIDs = new CFMutableDictionary(),
     return [self compare:aString options:CPCaseInsensitiveSearch];
 }
 
+// This is for speed
+var CPStringNull = [CPNull null];
+
 /*!
     Compares the receiver to the specified string, using options.
     @param aString the string with which to compare
@@ -504,6 +508,10 @@ var CPStringUIDs = new CFMutableDictionary(),
 */
 - (CPComparisonResult)compare:(CPString)aString options:(int)aMask
 {
+    if (aString === nil) return CPOrderedDescending;
+
+    if (aString === CPStringNull) [CPException raise:CPInvalidArgumentException reason:"compare: argument can't be 'CPNull'"];
+
     var lhs = self,
         rhs = aString;
 

Tests/Foundation/CPStringTest.j

@@ -566,4 +566,10 @@
     [self assert:"This is a test none" equals:[noneString stringByTrimmingCharactersInSet:set]];
 }
 
+- (void)testCompareWithNil
+{
+    [self assert:CPOrderedDescending equals:[@"Objective-J" compare:nil]];
+    [self assertThrows:function () { [@"Objective-J" compare:[CPNull null]] }];
+}
+
 @end