Revert "If HTTP charset header is empty, do not append content encoding ...

...to Content-Type header"

This reverts commit 3cc0e3fe3d33bb9dbd8357dd3430db7d507f4487.
This commit break globalization responseEncoding setting
see bug 23810 in xamarin bugzilla

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>