Character conversion from Source to Rich text within a PlantUML block doesn't work properly

Summary

When editing a wiki page and switching tab between Source and Rich text some characters within a plantuml block are not converted properly. Example: the characters æøå become æøå. Reverting to Source doesn't fix the conversion.

plantuml_source

plantuml_richtext

Steps to reproduce

1- In a GitLab.com project, click wiki on the left bar menu then Create a new page

2- In the text section copy the code block :

```plantuml
A -> B : Text with norwegian characters: æøå
```

3-Click on Rich text, the conversion of æøå is wrong.

Example Project

What is the current bug behavior?

The characters æøå (within a plantuml block) becomes æøå when clicking on Rich text.

What is the expected correct behavior?

The characters æøå (within a plantuml block) should stay the same when clicking on Rich text.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check 
(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)


(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)


(we will only investigate if the tests are passing)

Possible fixes

Edited by Segolene Bouly