Aws::S3::PresignedPost does not allow for extra headers

[desheikh]

S3 presigned posts allow for arbitrary headers if they are included in the policy but Aws::S3::PresignedPost does not allow for it.

For example when trying to allow for an arbitrary "x-requested-with" header.

This seems to be a regression from:
#297

[awood45]

Good point, it does look like we had this in V1. Taking this as a feature request, I'll take a look at copying over the old solution.

[cjyclaire]

This has been tracked in backlog to be prioritized : )

[mullermp]

Reopening - deprecating usage of Feature Requests backlog markdown file.