Use displayVersionString instead of versionString for OS version mism…

[samschott]

This PR fixes an issue where user-facing error messages about an operating system version mismatch (too new / too old) would show the versionString instead of the displayVersionString .

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Only bug fixes to regressions or security fixes are being backported to the 1.x (master) branch now. If you believe your change is significant enough to backport, please also create a separate pull request against the master branch.

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

(Describe all the cases that were tested)

macOS version tested: 10.13

[zorgiepoo]

Thanks. I now realize the documentation for displayVersionString is wrong and it shouldn't be nullable, which I'll fix separately.

[samschott]

Ah, indeed, I overlooked that it's marked as nullable. Though in practice, it looks like it is always set:

Sparkle/Sparkle/SUAppcastItem.m

Lines 526 to 530 in 8ea4d70

	if (shortVersionString) {
	_displayVersionString = [shortVersionString copy];
	} else {
	_displayVersionString = [_versionString copy];
	}

[zorgiepoo]

Indeed fixing that in #2139