Take care of 403 Forbidden HTTP error

Return AccessDenied error to client
instead of default InvalidURI when subsequents
request in pipeline return 403 error.
Fixes for canonicalize_url, object ACL query, signed URLs
Issue #49 - fixed Date and ExpireDate checking logic
Cleaned up PEP8 and Flake8 formatting errors.
canonical_string refactoring + fix typing in middleware comment
Merge pull request #53 from vitoordaz/master

canonical_string refactoring + fix typing in middleware comment