Set text/plain content-type on Micropub create response

[jlelse]

This fixes compatibility with the Sparkles micropub client (https://sparkles.sploot.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8f2ce2a) 83.30% compared to head (3508624) 83.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   83.30%   83.31%   +0.01%     
==========================================
  Files           9        9              
  Lines        1000     1001       +1     
==========================================
+ Hits          833      834       +1     
  Misses        117      117              
  Partials       50       50              

Files	Coverage Δ
micropub/handler.go	87.37% <100.00%> (+0.06%)	⬆️

[hacdias]

I will approve because I don't think it'll hurt. However, I think it's quite odd that that client is checking for the content-type on responses that are not expected to have any content themselves. Maybe it'd be good to open an issue with them too.

[jlelse]

CC @benjifs