[TIMOB-6828] Setting value as nil removes header from request

Fix for [TIMOB-6828](https://jira.appcelerator.org/browse/TIMOB-6828)
Can you please use Objective-C literals and add debug logging. Something like this:

```
    if (key == nil) {
        DebugLog(@"Ignore request to self.headers[%@] = %@", key, value);
        return;
    }
    
    if (value == nil) {
        DebugLog(@"Remove key %@ from self.headers (previous value of self.headers[%@] was %@", key, key, value);
        [self.headers removeObjectForKey:key];
    } else {
        DebugLog(@"self.headers[%@] = %@", key, value);
        self.headers[key] = value;
    }
```