Add support to ADD to preexistent headers

Currently the documentation specifies:

Existing response headers with the same name are always overridden. If you want to add headers incrementally, use the standard add_header directive instead.

But the add_header has the drawback that it removes ALL previously defined add_headers in upper sections.

It could be very useful to add parameters to an already defined header. For example, we can have a global Vary, but need to add more itens just inside a location. Or we may want to expand headers received from a proxy.

No files, tests, or entry points are named. Start by locating the module's handling of ADD and existing add_header inheritance, then clarify how additions to an inherited Vary header and proxy-provided headers should behave. Done means the intended syntax and behavior are defined and covered by tests.