fix bug with lower case units in opening fits file

[nabobalis]

Since allowing people to actual units is too far for sunpy.

[Cadair]

It's very sunpy.

[pep8speaks]

Hello @nabobalis! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers!

Comment last updated at 2022-03-16 22:44:13 UTC

[nabobalis]

Wonderful 10/10

[sunpy-backport]

The backport to 3.0 failed:

Commits ["caae6a38c592108470783d11df95bbd8ec823305","a2843c57f78943eafb4d565e9d5b96ff01f4e815","dfd6cd62ccd598bb5b9daf335521214d85d66c56","23deb0b2f43f5c34625cbe3bcc922bf4c775b2d0","259cdb7261a6e40e09700638631e1142fde7d467","3db4b725dd0b6791f4a70c58e165d5d6c2d0cdd6"] could not be cherry-picked on top of 3.0

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport 3.0
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick caae6a38c592108470783d11df95bbd8ec823305 a2843c57f78943eafb4d565e9d5b96ff01f4e815 dfd6cd62ccd598bb5b9daf335521214d85d66c56 23deb0b2f43f5c34625cbe3bcc922bf4c775b2d0 259cdb7261a6e40e09700638631e1142fde7d467 3db4b725dd0b6791f4a70c58e165d5d6c2d0cdd6
# Create a new branch with these backported commits.
git checkout -b backport-5970-to-3.0
# Push it to GitHub.
git push --set-upstream origin backport-5970-to-3.0
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is 3.0 and the compare/head branch is backport-5970-to-3.0.

[nabobalis]

The function was not changed in 3.0, only 3.1