ImmutableMappable init fix

[itruf]

I've started research to fix warning mentioned here:
#1043 (comment)

If you try to init ImmutableMappable with JSONString - swift shows warning that this method has no calls to throwing functions.

I found that:

• ImmutableMappable inherits from BaseMappable
• ImmutableMappable has an extension with init methods which throws (like init(JSONString: String, context: MapContext? = nil) throws)
• BaseMappable has an extension with optional init methods (like init?(JSONString: String, context: MapContext? = nil))

It causes crashes in some cases.

For example:
let response = try? MyModel(JSONString: jsonString) will crash if MyModel is ImmutableMappable and throws in init method.

I'm not sure but it can be caused by selecting optional init from BaseMappable which not marked with throws.

[itruf]

@tristanhimmelman I'm not pretty sure about library architecture. Hope that you will find some time to check my PR.

[itruf]

@tristanhimmelman it's a critical problem with ImmutableMappable. Please check it out.

[itruf]

@tristanhimmelman I'm really sorry for pushing you but it's the very important PR for the project. I hope that you will take a look at it.

[itruf]

Thank you @tristanhimmelman

[murad1981]

this update caused many crashes in my app where I init an ImmutableMappable from a json string and from a json dict, both now cause runtime crash.

[itruf]

@murad1981 can you show an example of your model and a JSON string caused crash? I think that it's correct behavior if you init an ImmutableMappable model with an incorrect JSON.