Steps to Reproduce

1. Inicialize http service in mac
2. Send post Content-Type = application/json on the header and json object in the body.
   For Content-Type = text/plain this error is not present

Current Behavior

On which platforms did you notice this

[X] macOS
[ ] Linux
[ ] Windows

Version Used:
Mojave 10.14.2

Mono JIT compiler version 5.16.0.221 (2018-06/b63e5378e38 Mon Nov 19 18:08:09 EST 2018)
Visual studio community 2017
Same tested Mono JIT compiler version 5.18.0.225 (2018-08/bac9fc1f889 Fri Dec 21 11:33:29 EST 2018)
PD: Same proyect found on windows with iis...