remove toLowercase on RFC2119 MUST/SHOULD terms

[UNTESTED]

Hi Robin,

RFC2119 terms are supposed to be all uppercase when printed as plain text. The toLowerCase here causes plain text output to be incorrect (try copy/paste or links dump), which is particularly annoying when pasting requirements into email.

However, since I don't know why they are being made lowercase, and I don't have an easy way to test it, this change may break something else down the line.  Is it because of an all-caps font/style? I'd rather have the content be correct and stick with the regular font/style.