SPDX license matching is incorrectly case-sensitive

[DaveRandom]

My composer.json:

{
  "name": "vendor/package",
  "license": "mit"
}

Output of composer diagnose:

Checking composer.json: WARNING
License "mit" is not a valid SPDX license identifier, see https://spdx.org/licenses/ if you use an open license.
/** stuff */

According to section 4 of the SPDX license matching guidelines:

All upper case and lower case letters should be treated as lower case letters

This issue is also causing packagist to fail updating from webhooks and manually, for example when I press "Update" here I get an error message stating:

Skipped branch master, Invalid package information:
License "mit" is not a valid SPDX license identifier, see https://spdx.org/licenses/ if you use an open license.
If the software is closed-source, you may use "proprietary" as license.

[phadej]

Note: spdx/spdx-spec#63 The issue is still open.