[v2] Save fail when using bind() in model constructor

[svobik7]

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When trying to update a model (previously fetched from server) I am getting TypeError: Cannot read property 'referenceDef' of undefined

This is because bound methods are included in Object.keys() but does not exists in datx fields. See below where code failes:

Method is bound in class constructor like this:

• datx version - 2.0.0-beta.7
• datx-jsonapi version - 2.0.0-beta.7

Steps to reproduce:

1. add class method dummy() {} to your json api model
2. bound this method in model constructor this.dummy = this.dummy.bind(this)
3. save model

[DarkoKukovec]

This fix was published in version 2.0.0-beta.8
Thanks for the contribution!