Fix new SSR API

[ciscorn]

Some fixes for the new SSR implementation:

• The component resolver should accept camelCase and PascalCase names in options.components
• no-prefetch is broken
• prefetch: true is not documented but should be allowed for the backward compatibility
• On making fake instances:
  • localData(vm) → localData.call(vm, vm)
  • Inject computed properties, methods, $data, $props, etc.
  • Use Object.assign instead of spread properties (which may cause a syntax error in some environments such as the latest Nuxt)

[Akryum]

Awesome! Someone picking up my work while I'm on vacations! 😻
Thanks for your PR! 👍