Reverted "Replaced Response.content_{type,length} with Response.set_content_{type,length}". Using setter to avoid breaking `Response.content_{type,length} = ...`

[masipcat]

Closes https://github.com/plone/guillotina/issues/920

[masipcat]

This PR fixes this:

https://github.com/plone/guillotina/blob/67b221f811732b6eaaacd981bc0fbbcd65e79cd8/guillotina/api/files.py#L59

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@36e44c8). Click here to learn what that means. The diff coverage is 85.8%.

@@           Coverage Diff            @@
##             master    #923   +/-   ##
========================================
  Coverage          ?   94.5%           
========================================
  Files             ?     333           
  Lines             ?   29429           
  Branches          ?       0           
========================================
  Hits              ?   27787           
  Misses            ?    1642           
  Partials          ?       0

Impacted Files	Coverage Δ
guillotina/files/manager.py	100% <ø> (ø)
guillotina/tests/test_api.py	99.6% <100%> (ø)
guillotina/json/deserialize_content.py	92.4% <100%> (ø)
guillotina/response.py	87.9% <50%> (ø)