fix(VDataTable): display non-string item values

[jacekkarczmarczyk]

Motivation and Context

fixes #8554

How Has This Been Tested?

visually, jest

Markup:

<template>
  <v-data-table
    :headers="[{value: 'foo'}]"
    :items="[{foo: 'a'}, {foo: true}, {foo: false}, {foo: new Date()}, {foo: [1,2]}, {foo: 123}]"
  />
</template>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)

[TravisBuddy]

Hey @jacekkarczmarczyk,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: eeec8c80-f47d-11e9-9e2b-01a46d152451

blah

[johnleider]

Abstract function into shared util for tables. Write specific test for it and remove the component ones.

[johnleider]

Pulse check.

[jacekkarczmarczyk]

Abstract function into shared util for tables. Write specific test for it and remove the component ones.

@johnleider not sure if it's a good idea, if someone removes usage of this function from the data table component test will still pass. Also it's just a tiny pieces of code used in only 2 places, so no much gain with extracting it out