NSInvocation+OCMAdditions: don't retain deallocating objects.

Solves erikdoe#346.
`allowsWeakReference` returns `NO` if the receiver is during
deallocation. Not retaining deallocating objects would prevent crashing
later when the retained arguments are released - including the
previously deallocated objects.
There are some classes that can never have weak references - these would
also not be retained which might limit some OCMock functionality.

Source/OCMock/NSInvocation+OCMAdditions.m

@@ -66,7 +66,10 @@ - (void)retainObjectArgumentsExcludingObject:(id)objectToExclude
     NSMutableArray *retainedArguments = [[NSMutableArray alloc] init];
 
     id target = [self target];
-    if((target != nil) && (target != objectToExclude) && !object_isClass(target))
+    // We don't want to retain a currently deallocating target, we check this by calling
+    // allowsWeakReference which returns NO in exactly this case.
+    if((target != nil) && (target != objectToExclude) && !object_isClass(target) &&
+       [target allowsWeakReference])
     {
         // Bad things will happen if the target is a block since it's not being
         // copied. There isn't a very good way to tell if an invocation's target
@@ -109,7 +112,9 @@ - (void)retainObjectArgumentsExcludingObject:(id)objectToExclude
                     // case do not retain the argument. Note: Even though the type is class the
                     // argument could be a non-class, e.g. an instance of OCMArg.
                 }
-                else
+                // We don't want to retain a currently deallocating object, we check this by calling
+                // allowsWeakReference which returns NO in exactly this case.
+                else if([argument allowsWeakReference])
                 {
                     [retainedArguments addObject:argument];
                 }