S3 meta data  handle plus sign

Previously, there was no way to reliably store and then retrieve
metadata values that contained plus signs. `quote_plus` left existing
pluses as they were while encoding spaces as "+". This would turn a
value like "x+ +x" into "x+++x" which upon retrieval from S3 using boto
would be decoded into "x   x".

This change allows storage of plus characters "+" in s3 key metadata
values.