Fix problem when uploading file using form-data in jersey2 client

Created by: JFCote

Here is the problem:

• When uploading a file with "multipart/form-data" and the jersey2 client, we receive this error:

multipart/form-data not yet supported for serializeToString (http signature authentication)

This error doesn't make sense since we do not use http signature authentication in our project. After some digging, I discovered that since the OAuth has been added, the method serializeToString was added to support http signature authentication and this method do not support "multipart/form-data". To fix it, I just added the serializeToString call when we support http signature authentication.

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