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.

CHANGELOG.md

@@ -3,6 +3,9 @@ Airbrake Ruby Changelog
 
 ### master
 
+* Started filtering the context payload
+  ([#55](https://github.com/airbrake/airbrake-ruby/pull/55))
+
 ### [v1.1.0][v1.1.0] (February 25, 2016)
 
 * Fixed bug in Ruby < 2.2, when trying to encode components while filtering

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