Reduce log level of 501 Not Implemented responses

[tggreene]

For public facing websites it's not uncommon to have exploitation suites run random strings as methods in small batches of requests. This commit channels 501s away from errors logs into info logs as we may still want to know that unknown methods are being used, typically though it's a part of the normal course of operation and don't want it to fill logging with high priority messages and potentially fall through into alerting for some users.

(There is a potential here for allowing users to implement their own :error-handler in the yada context, but I figure that might be best separated into another PR. Also, I apologise if it's preferred that an issue be created beforehand, I figure better to have the fix in hand ¯\_(ツ)_/¯).