Add compliance with the X-Forwarded-Prefix proxy header

[a-menshchikov]

Fix #20

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.93%. Comparing base (0ade7b8) to head (a43db4f).

Files with missing lines	Patch %	Lines
src/Onelogin/AuthFactory.php	83.33%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/Onelogin/AuthFactory.php	95.00% <83.33%> (-5.00%)	⬇️

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