The SigV4 encoder kept '/' unescaped in object paths by swapping in a
"never matches" regex (/$^/) for query values, but that regex matches the
empty string, so every empty query value went out as "/": prefix= became
prefix=/ and ?policy became ?policy=/. The request then no longer matched
what was signed, and listing any bucket or reading its policy failed
against s3d. Bucket listing and creation carry no query, so they worked.
Verified end to end against s3d (list, folders, upload, download, public
policy, folder delete) and against the aws cli; two regression tests.
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