Add ability to send x-amz-headers with upload and complete

This is needed when using **x-amz-security-token**, which is a needed header
when using temporary security credentials.
See AWS doc for details:
http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html#UsingTemporarySecurityCredentials

Note: To get temporary security credentials I set x-amz-security-token in all three spots: xAmzHeadersAtInitiate, xAmzHeadersAtUpload, and xAmzHeadersAtComplete.

Temporary security credentials are useful when you want to use IAM roles and not provide your permanent keys.