Allow arbitrary options for presigned post.

Presigned Posts to S3 previously hard coded a set of allowable
parameters, this change permits users to send arbitrary params
which are then passed on to S3.

Resolves #250
Minor adjustment to previous presigned post change.

Had integration test regressions regarding some option types.
This change addresses that regression.
Remove unnecessary field name transformation.

In my previous change, forgot the removal of one field name
function call. This was inconsistent but didn't break tests.
Cleaning that up with this change.
Merge pull request #297 from aws/issue250

Allow arbitrary options for S3 presigned posts.