available_contexts returns [] instead of nil on error

Fix #212

Author: bootstraponline

lib/appium_lib/device/device.rb

@@ -16,7 +16,6 @@ module Device
       get:  {
         current_activity:   'session/:session_id/appium/device/current_activity',
         current_context:    'session/:session_id/context',
-        available_contexts: 'session/:session_id/contexts',
       }
     }
 
@@ -113,6 +112,13 @@ def extended(mod)
           pair.each_pair { |command, path| add_endpoint_method command, path, verb }
         end
 
+        add_endpoint_method(:available_contexts, 'session/:session_id/contexts', :get) do
+          def available_contexts
+            # return empty array instead of nil on failure
+            execute(:available_contexts, {}) || []
+          end
+        end
+
         add_endpoint_method(:app_strings, 'session/:session_id/appium/app/strings') do
           def app_strings language=nil
             opts = language ? { language: language } : {}