Allow plus in key names

[NickH-nz]

Keys containing the character Plus ("+"), fail the md5 signature check. For example a file with key: 5-ID+Rub+IgG+II+30221_01_2002_FR.docx

gof3r error: 403: "The request signature we calculated does not match the signature you provided. Check your key and signing method."

Related to the linked issue which fixed the colon (":") and at ("@") characters.

Originally posted by @ewold in https://github.com/rlmcpherson/s3gof3r/issues/110#issuecomment-346761148