Fix PlantUML diagram content header

[orlade-anz]

Fixes #888.

Changes proposed in this pull request:

• Add ~1 to the beginning of the content sent to PlantUML server.

This is not a perfect solution, since it will probably cause the reported error for users of older PlantUML versions. But it is better to break them than to break the version in production, so this is at least a progression. Someone should follow up to make it work it both cases.

Checklist:

• [ ] Added related tests
• [ ] Made corresponding changes to the documentation

[codecov[bot]]

Codecov Report

Merging #906 into master will increase coverage by 0.01%. The diff coverage is 89.18%.

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
+ Coverage   83.35%   83.36%   +0.01%     
==========================================
  Files          73       73              
  Lines       10609    10625      +16     
==========================================
+ Hits         8843     8858      +15     
- Misses       1432     1433       +1     
  Partials      334      334              

Impacted Files	Coverage Δ
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	:arrow_up:
pkg/importer/openapi.go	88.95% <80.00%> (-0.50%)	:arrow_down:
pkg/diagrams/diagutil.go	62.31% <100.00%> (ø)
pkg/importer/utils.go	88.23% <100.00%> (+0.35%)	:arrow_up:
...g/mermaid/integrationdiagram/integrationdiagram.go	86.66% <100.00%> (+2.29%)	:arrow_up:
pkg/mod/module.go	67.56% <100.00%> (+1.85%)	:arrow_up: