/usr/lib/mono/4.5/System.Net.Http.dll is in libmono-system-net-http4.0-cil 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/System.Net.Http/4.0.0.0__b03f5f7f11d50a3a/System.Net.Http.dll