Support for data structure that can not be used in the property list.

I want to use this as.
```
[Bugsnag notify: [NSException exceptionWithName:@"net.dealforest.exception" reason:@"sample" userInfo:nil]
       withData: @{ @"error": [NSError errorWithDomain:@"net.dealforest.demo" code:200 userInfo:nil] }
];
```
but this method call is failed.
and the developer will not be able to notice that it has failed.
The reason for this is using a data structure that is not supported in the property list.
(https://github.com/dealforest/bugsnag-cocoa/blob/master/bugsnag/BugsnagNotifier.m#L160)

so I think the unsported object is call `-[NSObejct description]`.

What do you think about this?

Regards best.