pithos: Fix missing 'Content-Disposition' in response

Since version 0.15 a specific content disposition type can be enforced in
requests for an object's content, using the 'disposition-type' optional
parameter.

Erroneously, if "disposition-type" is not provided, 'Content-Disposition'
header is not returned in the response.
pithos: Handle spaces in 'Content-Disposition'

Handle properly objects whose name contains spaces
pithos: Add tests for the 'Content-Disposition'