Make filters and additional network requests easier

[joeyparrish]

While working on a tutorial for asynchronous request filters, I found that the tutorial code was more complicated than it should be. The main problems were:

1. Converting between ArrayBuffer and string requires too much effort outside the library. An app/filter should be able to deal with request and response bodies either as ArrayBuffer or string.
2. Making additional network requests from the application level required all request fields to be provided. The NetworkingEngine should fill in defaults when fields are missing.