Fix issue with invalid callback scheme

[esjewett]

This fixes the issue raised in #18 with a minimal change, not resulting in a change to the actual value passed to callbackURLScheme.

Would also request that this be cherry-picked into a 2.x release as we are still using that version.

Thank you!

[esjewett]

@dpogue Sorry for the bother - just wanted to check if you might have a chance to look at this. Thanks!

[dpogue]

Sorry, I've had no time to look into this. I'm setting a reminder for myself to revisit this when I have some capacity in July.

[isc-elizawmatt]

@dpogue I also wanted to check in on the status of this PR. My team has also hit this issue, and the proposed fix does resolve it for ios 14.5+. As with @esjewett, we are currently using cordova-plugin-oauth@2.0.1, so would appreciate the change being cherry-picked to a 2.x version.

[viktor-shmigol]

Confirmed. These changes are fixed the crash issue in IOS 15.3.1
@dpogue dpogue and @esjewett esjewett could you please review and probably merge this PR?
Thanks!

[dpogue]

I've released v3.0.2 and v2.0.2 with a fix. I opted to go with just taking the scheme portion of the URL rather than URL encoding it, because that seems more in line with Apple's intention.