Add support for merging mixins

[adrienbaron]

A few libraries in Nuxt (vue i18n, Vuetify) uses mixins for Vue Meta.
Vue Meta for now doesn't support Mixins merging, resulting in conflicts when using both those libraries at the same time in Nuxt.

I will try to add support for mixins merging and do a pull request as offered by @atinux

See: nuxt/nuxt#3846

[adrienbaron]

I managed to add the feature this way:

• I register the optionMergeStrategies to be the same as created, meaning options get merged in an Array
• I added the ability to getComponentOption to merge a top level Array of options

Two things I had to do that I'm not sure about:

• I had to change the test allows for a custom array merge strategy in getComponentOption
  This tests was passing an Array as a top level option object, but we now assume that arrays at top levels means that we should merge it's content. So I changed the test to pass an object containing the array instead:

foo: [
  { name: 'flower', content: 'rose' }
]

became:

foo: {
  flowers: [
    { name: 'flower', content: 'rose' }
  ]
}

• I had to manually register the optionMergeStrategies in the test of getMetaInfo
  This is to allow testing the merging of mixins in this test
  See: adrienbaron@ad4d61c#diff-aad47313d609c07f1d9316d0918465a4R24

If those changes are OK I'll create the pull request :)

[atinux]

Agree to create the PR, would love to have @manniL review also. Great job!

[bdrtsky]

I am not sure, this option is working or not? I am trying to use mixin with head but it's not working

[adrienbaron]

@bdrtsky This PR was reverted because it was breaking other things (see: #269)

[philefstat]

I just wanted to share for others who come across this issue, who are using the head() function in a mixin - you can use the following to merge the result of the mixin:

let strategies = Vue.config.optionMergeStrategies
strategies.head = strategies.data

[Gvade]

I just wanted to share for others who come across this issue, who are using the head() function in a mixin - you can use the following to merge the result of the mixin:

let strategies = Vue.config.optionMergeStrategies
strategies.head = strategies.data

@philefstat At what place do you reassign the merge strategy? I can't use vue.config in nuxt.config.js as it doesn't accept function.