Fix error when redirecting to a non-http URL

[acelaya]

Closes #2060

Fix an error when trying to redirect to a non-http URL (like the ones used for deeplinks, for example).

This PR also adds a bunch of tests covering that, as this was a regression introduced in https://github.com/shlinkio/shlink/pull/1991/files#diff-abf169189b40a92b43093bda1780b27f63581b5fc9c4644fca78c5913773acd8

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.34%. Comparing base (293725f) to head (053e026).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2063   +/-   ##
==========================================
  Coverage      97.34%   97.34%           
  Complexity      1363     1363           
==========================================
  Files            251      251           
  Lines           4826     4827    +1     
==========================================
+ Hits            4698     4699    +1     
  Misses           128      128           

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