middleware: fix date header parse error log

[danielhochman]

This log message in middleware.go is not properly formatted because the format is being passed as the classification to Logf(classification Classification, format string, v ...interface{}).

Example output:
SDK 2021/02/01 22:20:44 failed to parse response Date Header value, got %!v(MISSING) parsing time "Mon, 1 Feb 2021 22:20:44 GMT" as "Mon Jan _2 15:04:05 2006": cannot parse ", 1 Feb 2021 22:20:44 GMT" as " "

Looking at the error and associated parsing code, it looks like an issue with some AWS endpoint not returning compliant timestamps that causes this warning (should technically be 01 Feb not 1 Feb). I can provide a fix for that in a separate PR with slightly more permissive Date header parsing if needed.

[skmcgrail]

@danielhochman out of curiosity, are you seeing this with a specific service or is this message getting logged for all service responses?

[danielhochman]

@skmcgrail looks like it's coming from the Kinesis API on ListStreams call though I can't be 100% sure

[danielhochman]

@skmcgrail I tested in three regions, us-east-1, us-west-1, and us-west-2. ListStreams in us-west-2 is reproducible and returns non-compliant timestamps, other regions were fine. Other service calls in us-west-2 also fine, e.g. EC2 DescribeInstances.

[skmcgrail]

Thank you for this PR @danielhochman , I've cut a tracking issue so that we can follow-up on supporting the datetime format Kinesis is sending package in the HTTP Date header. Thank you for providing the investigation details.