Option to use x-amz-date header instead of Date

I have noticed that with browserify / jspm the `Date` header gets stripped causing AWS to reject the request saying the signature does not match. It seems that `stream-http` will strip the `Date` header because it is "unsafe"

https://www.w3.org/TR/XMLHttpRequest/#the-setrequestheader%28%29-method

To work around this the client should have an option to set the `x-amz-date` header instead of `Date`. Is this reasonable?

Start by tracing how the client generates the Date header and includes it in AWS request signing, with the browserify/jspm and stream-http behavior in mind. Determine where an option for x-amz-date belongs; done means browser requests can use that header and AWS accepts the resulting signature, with coverage for both header modes.

Written by the indexing model from the issue text.