Prior to this commit, there was no distinction between explicitly setting
forward headers strategy to a value of NONE and not setting it at all.
This meant that in a cloud environment, a cloud provider was always checked
to see if it was active and using forward headers and there was no way to
prevent that.

This commit changes the default value of the property to null so that there
is a way to determine if the property was explicitly set to NONE.

Fixes gh-19333