[java][resteasy] fixes serialize list of files and deserialize file

[nicolasiannarilli]

fix serialize multipart with list of files : serialize a list with toString

fix deserialize byte and file : test on GenericType always return false

fix downloadFileFromResponse : prepareDownloadFile(response) return a temp file and copy failed with FileAlreadyExistException

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[wing328]

@nicolasiannarilli thanks for the PR. Is this ready for review and merge?