Add tests for previously untested functions:
- edns0.go: test supportedOptions function
- request.go: test address methods, protocol handling, and EDNS0
options
- writer.go: test ScrubWriter implementation
Improves overall package test coverage from 39.5% to 77.8%.
Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
