[bug?]: Webadmin POST text/plain to the server when login.

[OrvilleQ]

Relate to stalwartlabs/mail-server#722

Seems like when press the Sign in button of the webadmin login page, it will POST /api/oauth and /api/token with content type text/plain;charset=UTF-8

This behavior is not allowed by Modsecurity rule 920420, which restricts the content types allowed to be sent to the server as POST and ultimately results in a 403 Forbidden being returned, which appears to be how Webadmin detects the missing TOTP code and ultimately results in the TOTP code page being displayed. stalwartlabs/mail-server#722 (comment)

Given that Modsecurity is used in many WAF programs, perhaps this behavior should be considered a bug and fixed? I'm also not sure how many POST requests are actually sent as text/plain, What do you think?