load session data for merge!
load session data for merge!

ID#prepare_session calls merge! on the newly-created SessionHash, but
this method is not overridden to parse existing data. As such, any
previous session data passed in from an earlier middleware is discarded.

For me, this was breaking Rack::Test while testing a Sinatra app.

getaroom/rack,getaroom/rack,getaroom/rack