The current code determines the header order for the
"string-to-sign" payload by sorting on the concatenation
of headers and values, whereas it should only happen on the
key.
During multipart uploads, since `x-amz-copy-source-range` and
`x-amz-copy-source` headers are present, V2 signatures fail to
validate since header order is swapped.
This patch reverts to the expected behavior.
Signed-off-by: Pierre-Yves Ritschard <pyr@spootnik.org>
