Incorrect 2020 UI exemption logic

[martinholmer]

Since pull request #2573 was merged into the master branch on March 15, 2021, the 2020 UI exemption has reduced taxable income but left AGI unchanged. Leaving AGI unchanged is not correct as a reading of 2020 Form 1040 and its Schedule 1 makes clear. The incorrect AGI amount for some people can cause incorrect tax calculations when AGI is used for some purpose (like to phase-out credits).