Test for 'Age header set by Edge'. Also includes code to cache IP address.

This adds tests to verify the behaviour of the 'Age' header. We discovered that Fastly resets this to '0', regardless of what origin provides.

In the process, it was also highlighted that we need some form of IP address caching - some tests require that we are at least talking to the same edge IP (and hence location), and DNS record timeout was causing an issue for us in this test. A custom `Dial` function has been created in 4a6c5cd to ensure we talk to the same IP address throughout the test suite.