Fix for logging interfering with multipart uploads

[marcelhamel]

A proposed solution for Issue #49, where an effort to log a multipart payload by creating a duplicate object inadvertently modifies the request payload itself. The result was that multipart requests to the Job API were sending a string meant for logging rather than a file, and this was somehow triggering the signature hash error message.

This is a one-line fix which should preserve the integrity of the request payload.

[expe]

Thanks @marcelhamel for the PR, just checked that this does fix the multipart uploads.