Reasons for creating this PR

Fix LCSH label lookups by making HTTP header parsing case-insensitive. See #1619
This PR backports the fix from Skosmos 3 to Skosmos 2.

Link to relevant issue(s), if any

• Closes Label lookups for LCSH mappings are no longer working #1619 (for Skosmos 2)

Description of the changes in this PR

Make LOC HTTP prefLabel header parsing case-insensitive in LOCResolver.php

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)