Call original debounced function with proper "this" context

[primeare]

Notable Changes

• metasync preserves debounced function's "this" context
• debounced function can be called with specific arguments (not only with the pre-populated ones)
• new changes are backward compatible with previous metasync API

Checks

• [X] tests and linter show no problems (npm t)
• [X] tests are added/updated for bug fixes and new features
• [X] code is properly formatted (npm run fmt)