Provide a protocol to the link to ExportRDF

[alex-mashin]

Provide a protocol to the link to ExportRDF, if $wgServer is protocol relative.

Fixes #99

[gesinn-it-gea]

I have updated the CI. Please rebase.

[alex-mashin]

I have updated the CI. Please rebase.

Rebased.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.43%. Comparing base (15887ad) to head (70db151).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/JsonLDSerializer.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #100   +/-   ##
=========================================
  Coverage     82.43%   82.43%           
  Complexity       80       80           
=========================================
  Files             7        7           
  Lines           205      205           
=========================================
  Hits            169      169           
  Misses           36       36           

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