Recursively convert MSObject to NSDictionary when serializing MSObject

[tobihagemann]

Fixes #27

As proposed in #27 (comment), this PR enhances the -[getSerializedDataWithError:] method in MSObject by recursively converting MSObjects to NSDictionary.

Since I have no idea how the models are actually generated, I didn't propose a solution that would make the dictionaries serializable from the get-go.

[msftclas]

All CLA requirements met.

[tritter]

I also stumbled on this, but also fixed enum types thanks to the fix of @tobihagemann. He may first merges my branch: https://github.com/tritter/msgraph-sdk-objc-models

[tobihagemann]

@tritter Done! Cherry-picked from your commits because I didn't want to merge unnecessary changes from master to my fix-27 branch.

[liambolling]

This works for me

[S2V]

Recursively convert to NSDictionary is missed - MS really? It looks like that should be released ASAP in the 1.3.1 version.

[S2V]

@tobihagemann Also, I guess it should be correct to update the method getDictionary of the MSObject class to something like that:

-(NSMutableDictionary *)getDictionary
{
	return [[self recursivelyConvertObjectsToDictionariesInDictionary:self.dictionary] mutableCopy];
}

because the classes which inherit MSObject just put(set) the not serialized values into the mutable dictionary.

[tobihagemann]

I haven't thought about this PR for a while now. You could be right but I have no idea what side-effects this would have. As I've stated in the beginning:

Since I have no idea how the models are actually generated, I didn't propose a solution that would make the dictionaries serializable from the get-go.

Edit: To be more specific, -[MSObject getDictionary] is a public method and the documentation just states that it returns the underlying dictionary. I guess that's what the method is currently doing. To improve that and to prevent breaking API changes, you would have to think about the side-effects of such a change.