Fix canonical request generation #98

Hi,

One osc-cli user reached me about a bug we may have in signature process:

osc-cli --profile profile fcu DescribeVolumes --Filter '[{"Name":"volume-id", "Value":"vol-082158a8 test"}]'

The space (" ") char is transformed in + which should actually be url-encoded in %20

FCU is pretty flexible on this but Outscale API may be more close to canonical request specification (In 3rd step, check double escape and header order)