Remove the X- prefix on headers

[jshayes]

Using the X- prefix was the old convention, but do to issues that it causes the use of X- prefixed headers has been deprecated.

We could remove the X- prefix on out headers, but we should probably make sure the resulting header name it not something too generic that could likely conflict with other headers out there. We may want to introduce our own prefix.

[jshayes]

Maybe these headers should be configurable like the algorithm and signature headers.