### Please describe the problem.

I have an existing Git annex repository with an Amazon S3 remote. I don't have the original credentials for this S3 account any more, but I would like to enable this remote in a fresh clone. When using the new IAM credentials, I get a SignatureDoesNotMatch error.

### What steps will reproduce the problem?

    [ollie@nixos:/media/music]$ export AWS_ACCESS_KEY_ID="redacted"
    
    [ollie@nixos:/media/music]$ export AWS_SECRET_ACCESS_KEY="redacted"
    
    [ollie@nixos:/media/music]$ git annex enableremote S3musicencrypted
    enableremote S3musicencrypted (checking bucket...) (creating bucket in EU...) git-annex: S3Error {s3StatusCode = Status {statusCode = 403, statusMessage = "Forbidden"}, s3ErrorCode = "SignatureDoesNotMatch", s3ErrorMessage = "The request signature we calculated does not match the signature you provided. Check your key and signing method.", s3ErrorResource = Nothing, s3ErrorHostId = Just "jjTIOt9HhXtYTYTHQB7a7CryvCVwZI2nW2EoR/GcF5QxlNf//qDd5ySLnlBiU3BDN4KzZuI2U8w=", s3ErrorAccessKeyId = Just "redacted", s3ErrorStringToSign = Just "PUT\n\n\nTue,  3 Feb 2015 19:36:58 GMT\n/s3musicencrypted-a7e22956-5ce2-11e2-97a5-6f33220ffbf3/"}


### What version of git-annex are you using? On what operating system?

    [ollie@nixos:/media/music]$ git annex version
    git-annex version: 5.20150113
    build flags: Assistant Webapp Webapp-secure Pairing Testsuite S3 WebDAV Inotify DBus DesktopNotify XMPP DNS Feeds Quvi TDFA CryptoHash TorrentParser
    key/value backends: SHA256E SHA1E SHA512E SHA224E SHA384E SKEIN256E SKEIN512E SHA256 SHA1 SHA512 SHA224 SHA384 SKEIN256 SKEIN512 WORM URL
    remote types: git gcrypt S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository version: 5
    upgrade supported from repository versions: 0 1 2 4

    [ollie@nixos:/media/music]$ uname -a
    Linux nixos 3.18.3 #1-NixOS SMP Thu Jan 1 00:00:01 UTC 1970 x86_64 GNU/Linux

    [ollie@nixos:/media/music]$ cat /run/current-system/nixos-version 
    15.05pre-70b398d
