Apply fixes from StyleCI

[voku]

This pull request applies code style fixes from an analysis carried out by StyleCI.

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This change is 

[codecov-io]

Codecov Report

Merging #4 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #4   +/-   ##
=========================================
  Coverage     89.86%   89.86%           
  Complexity      119      119           
=========================================
  Files             6        6           
  Lines           286      286           
=========================================
  Hits            257      257           
  Misses           29       29

Impacted Files	Coverage Δ	Complexity Δ
src/Purl/Url.php	93.28% <100%> (ø)	60 <19> (ø)	⬇️
src/Purl/AbstractPart.php	66.07% <100%> (ø)	24 <1> (ø)	⬇️

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[coveralls]

Coverage remained the same at 91.88% when pulling 2f7aa02 on analysis-zYa1R0 into 9e39483 on master.