cli: improve how reana spec is handled in `download` and `upload` commands

[audrium]

After introducing missing workflow spec schema to get_workflow_specification endpoint, reana-client started to include and set all the missing properties to None in the response of this endpoint. This affected upload and download commands since internally this endpoint is used to retrieve the spec and get the files to upload/download. This PR fixes this issue

[codecov-commenter]

Codecov Report

Merging #635 (1c01f3e) into master (e717555) will not change coverage. The diff coverage is 25.00%.

Additional details and impacted files

[](https://codecov.io/gh/reanahub/reana-client/pull/635?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) ```diff @@ Coverage Diff @@ ## master #635 +/- ## ======================================= Coverage 56.65% 56.65% ======================================= Files 21 21 Lines 2180 2180 ======================================= Hits 1235 1235 Misses 945 945 ``` | [Impacted Files](https://codecov.io/gh/reanahub/reana-client/pull/635?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) | Coverage Δ | | |---|---|---| | [reana\_client/cli/files.py](https://codecov.io/gh/reanahub/reana-client/pull/635/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub#diff-cmVhbmFfY2xpZW50L2NsaS9maWxlcy5weQ==) | `76.57% <25.00%> (ø)` | |

[mdonadoni]

minor: what do you think about adding the description of the PR to the commit message?