Fix SignedHeaders
Fix SignedHeaders

Without this patch:
Signed headers: accept-encoding;content-type;host;user-agent;x-amz-content-sha256;x-amz-date;x-amz-acl;x-amz-meta-created;x-amz-meta-owner;x-amz-storage-class

With this patch:
Signed headers: accept-encoding;content-type;host;user-agent;x-amz-acl;x-amz-content-sha256;x-amz-date;x-amz-meta-created;x-amz-meta-owner;x-amz-storage-class

The sort was done on headers with upper/lower case, this patch fix this
beahiavor.

mistio/libcloud,apache/libcloud,apache/libcloud,mistio/libcloud,mistio/libcloud,apache/libcloud