response: drain response body on error
response: drain response body on error

With the older version of Swift, proxy-logging prints 499 errors if the
response body remains untouched.  This patch fixes it.

Change-Id: I9472cd539b344a988be3b294361e075d9760904e

stackforge/swift3,swiftstack/swift3-stackforge,swiftstack/swift3-stackforge,KoreaCloudObjectStorage/swift3,KoreaCloudObjectStorage/swift3,stackforge/swift3,tumf/swift3,tumf/swift3