filters/keys: don't try to filter already filtered value

Fixes #276 (Specifying `blacklist_keys` with `whitelist_keys` can result in
error)

lib/airbrake-ruby/filters/keys_filter.rb

@@ -131,6 +131,7 @@ def validate_patterns
 
       def filter_context_key(notice, key)
         return unless notice[:context][key]
+        return if notice[:context][key] == FILTERED
         return filter_hash(notice[:context][key]) unless should_filter?(key)
 
         notice[:context][key] = FILTERED

spec/notifier_spec/options_spec.rb

@@ -226,5 +226,23 @@ def expect_a_request_with_body(body)
         include_examples 'sent notice', environment: :development
       end
     end
+
+    describe ":blacklist_keys" do
+      # Fixes https://github.com/airbrake/airbrake-ruby/issues/276
+      context "when specified along with :whitelist_keys" do
+        it "sends a notice" do
+          params = {
+            blacklist_keys: %i[password password_confirmation],
+            whitelist_keys: [:email, /user/i, 'account_id']
+          }
+          airbrake = described_class.new(airbrake_params.merge(params))
+          notice = airbrake.build_notice(ex)
+          notice[:context][:headers] = 'banana'
+          airbrake.notify_sync(notice)
+
+          expect(a_request(:post, endpoint)).to have_been_made
+        end
+      end
+    end
   end
 end