Allow KWCaptureSpy to capture nil block arguments

[ricketson]

When trying to capture a block argument that is nil, KWCaptureSpy throws an exception and claims that the argument was never captured.

I believe that the error is due to this method in KWCaptureSpy.m:

- (void)object:(id)anObject didReceiveInvocation:(NSInvocation *)anInvocation {
    if (!_argument) {
        NSMethodSignature *signature = [anInvocation methodSignature];
        const char *objCType = [signature messageArgumentTypeAtIndex:_argumentIndex];
        if (KWObjCTypeIsObject(objCType) || KWObjCTypeIsClass(objCType)) {
            void* argumentBuffer = NULL;
            [anInvocation getMessageArgument:&argumentBuffer atIndex:_argumentIndex];
            id argument = (__bridge id)argumentBuffer;
            if (KWObjCTypeIsBlock(objCType)) {
                _argument = [argument copy];
            } else {
                if(argument == nil) {
                    _argument = [KWNull null];
                } else {
                    _argument = argument;
                }
            }
        } else {
            NSData *data = [anInvocation messageArgumentDataAtIndex:_argumentIndex];
            _argument = [KWValue valueWithBytes:[data bytes] objCType:objCType];
        }
    }
}

Specifically,

if (KWObjCTypeIsBlock(objCType)) {
    _argument = [argument copy];
} else {
    if(argument == nil) {
        _argument = [KWNull null];
    } else {
        _argument = argument;
    }
}

should be...

if (argument == nil) {
    _argument = [KWNull null];
} else if (KWObjCTypeIsBlock(objCType)) {
    _argument = [argument copy];
} else {
    _argument = argument;
}

Since the internal system that KWCaptureSpy uses to tell whether an argument is captured or not is whether it is equal to nil, not setting _argument to [KWNull null] when argument is a block and nil causes it to think that the argument was never captured.

[modocache]

Looks solid, @mricketson! Could you please add a unit test that demonstrates this fixes the problem (i.e.: one that passes with this change but fails without it)?

[sozorogami]

@modocache How's that? If you prefer, I could leave the old test alone and implement a stand-alone test for nil blocks.

[modocache]

@sozorogami Could you add your commit in #555 on top of @mricketson's commit? CI fails in #555 because your test fails without the implementation code in this PR's commit, 74481b2. You can pull down @mricketson's branch using:

git checkout -b mricketson-master master # do all this on a new branch
git pull git@github.com:mricketson/Kiwi.git master # pull @mricketson's work down onto that new branch

Then you can put your commit on top of it using:

git checkout mricketson-master # make sure you're on the new branch
git cherry-pick 5fac65b # apply your commit from #555