filters: filter context payload

A user reported that it's not possible to filter context/user. It's a
reasonable request given that the user information has been moved to
context recently.

lib/airbrake-ruby/filters.rb

@@ -5,6 +5,6 @@ module Filters
     ##
     # @return [Array<Symbol>] parts of a Notice's payload that can be modified
     #   by various filters
-    FILTERABLE_KEYS = [:environment, :session, :params].freeze
+    FILTERABLE_KEYS = [:environment, :session, :params, :context].freeze
   end
 end