aws/defaults: Fix request metadata headers causing signature errors

Fixes the SDK's adding the request metadata headers in the wrong
location within the request handler stack. This created a situation
where a request that was retried would sign the new attempt using the
old value of the header. The header value would then be changed before
sending the request.

This moves the request invocation id to only occur during build,
maintaining its value across all requests attempts. Also moves request
retry metadata header to be before sign, and after build. This ensures
that a new value for each attempt can be set, and included in each
request attempt.

Fix aws#533
Fix aws#521

aws/defaults/defaults.go

@@ -70,10 +70,10 @@ func Handlers() aws.Handlers {
 	handlers.Validate.AfterEachFn = aws.HandlerListStopOnError
 	handlers.Build.PushBackNamed(SDKVersionUserAgentHandler)
 	handlers.Build.PushBackNamed(AddHostExecEnvUserAgentHander)
+	handlers.Build.PushFrontNamed(RequestInvocationIDHeaderHandler)
 	handlers.Build.AfterEachFn = aws.HandlerListStopOnError
 	handlers.Sign.PushBackNamed(BuildContentLengthHandler)
-	handlers.Send.PushFrontNamed(RequestInvocationIDHeaderHandler)
-	handlers.Send.PushFrontNamed(RetryMetricHeaderHandler)
+	handlers.Sign.PushFrontNamed(RetryMetricHeaderHandler)
 	handlers.Send.PushBackNamed(ValidateReqSigHandler)
 	handlers.Send.PushBackNamed(SendHandler)
 	handlers.Send.PushBackNamed(AttemptClockSkewHandler)