refactor(object-spread): use spread instead of assign

[liangchunn]

Just following up from #166 and #169:

Since we've already dropped support for Node 6 in #166, object spread properties can now be safely used. (As briefly discussed here)

In .eslintrc.json, "experimentalObjectRestSpread": true and "ecmaVersion": 6 should be kept, since Node 8 supports 99% of ES2015 features, and object-spread is just a ES2018 feature, but is fully supported in Node 8 and above. (Also mentioned in #169, here)

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Note: This PR does not introduce any functionality changes

[johnnyreilly]

Nice - either way; we transpile typescript down to es2015 so the object-spread can happily be used since (I believe) it gets transpiled into Object.assign anyway. But yeah - completely happy with these changes. Do you want update the version number in package.json / add an entry to CHANGELOG.md and I'll merge this. This can go out as alpha.1 I guess.

[liangchunn]

we transpile typescript down to es2015 so the object-spread can happily be used since (I believe) it gets transpiled into Object.assign anyway.

Yep! This only impacts test files, and just making them look nicer. Either way we could push this as alpha, or not at all. (Up to you, I don't really mind 😄)

[johnnyreilly]

Thanks!