Safe HTTP methods should be retryable

In HttpTransact::is_request_retryable, nothing seems to be retryable if you have sent any bytes. Following the RFCs, the default behaviour should allow safe (and also idempotent) method requests to be retried regardless of thether bytes were sent.

"safe" methods (https://tools.ietf.org/html/rfc7231#section-4.2.1): GET HEAD

From conversations, it sounds like the ideal approach is to create a config option (which is transaction overrideable) which allows you to define the list of methods which are retryable (which wouldn't be limited to the well-known methods inside ATS).