Add toJSON method to FeathersVuexModel class

[NickBolles]

Steps to reproduce

• Tell us what broke. The more detailed the better.
• If you can, please create a simple example that reproduces the issue and link to a gist, jsbin, repo, etc.

I'm not exactly sure what is causing this, besides that I'm doing SS authentication, and trying to store the

‼ warn Cannot stringify arbitrary non-POJOs FeathersVuexModel

I think this could be resolved by adding a toJSON method on the class. Plus in-general it's a good practice.

nuxt-contrib/devalue#1

[DreaMinder]

So is this resolved by adding toJSON? Because I still got warnings
"feathers-vuex": "^1.6.2",

[NickBolles]

@DreaMinder I'm not sure that there's been another release to non since this change
@marshallswain do you know?

[marshallswain]

Lots of them in the pre channel

npm i feathers-vuex@pre