feat: allow creating new instances

[mercs600]

This PR resolves #286.

I added new method for create copy of $axios instance based on default settings.
After that the copy is independent and may get new options so we can extend it.
Right now we can't use nuxt/axios in our module, because we won't override axios instance created by user.

How it works ?
$axios.create method returns new Axios instance, you can pass additional options to merge.

const newInstance = this.$axios.create({
  headers: {
    common: {
      Accept: 'text/plain, */*'
    }
  }
})

After that you can use methods from this module, without override base $axios instance.

newInstance.setBaseURL('https://yourapi.com')

I also extended test/fixture/pages/ssr.vue to present how the new instance behaves.

@pi0 Have a look at it please.

[codecov]

Codecov Report

Merging #306 into dev will not change coverage.
The diff coverage is n/a.

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  Coverage   100%   100%           
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  Files         1      1           
  Lines        30     30           
  Branches     13     13           
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  Hits         30     30

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[pi0]

Thanks for initial PR 👍

[mercs600]

I removed ctx here, because it is not used

[mercs600]

@pi0 Is it suitable to merge ? This is not breaking change, but it would be helpful to implement nuxt-axios in other modules. Let me know please :)

[nachogarcia]

Any news from this @pi0 ?
There's a MR in axios also axios/axios#2040

[pi0]

Awesome! Thanks!

[pi0]

Maybe we could document how to inject instance to CTX too?

[mercs600]

Sure, I can add it