Add test for AccessLogRecord.Emit, happy path

Also added a mockEmitter.New and used it where relevant

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>
Move creation of complete access logger to function

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>
AccessLogRecord.emit works without RouteEndpoint

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>
Log request even if proxy exits early or panics

[Finishes #63074062]

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>