Added cleaner method for creating HttpClient in test

[johnrengelman]

Some test cases for services or handlers require construction of a HttpClient instance that is normally injected by the server registry in a running application.
These tests would often utilize the HttpClient.of {} initialization the test case to create the instance.
As a result of #1581 and introducing the Netty based DNS resolver, this no longer works without either:

1. Explicitly wiring in an ExecController instance: HttpClient.of { it.execController(execController)}
2. Explicitly setting HttpClient.of { it.useJdkAddressResolver() }

A little additional syntax that creates an HttpClient with the JDK resolver (blocking) for test uses (which is almost always acceptabled) could help users in the migration.