The sequence of napi_open_callback_scope(),
napi_call_function(), and napi_close_callback_scope() functions calls
executes the provided JS code and all functions enqueued by
process.nextTick() and Promises during this execution.
This fixes two issues:
- values for mutiple header fields with the same name
passed as arrays were converted to string;
- the type of field value wasn't preserved as required
by specification.
