fix: use to_hash to expose @src

Author: Yuki-Inoue

lib/line/bot/event/base.rb

@@ -19,11 +19,14 @@ class Base
         def initialize(src)
           @src = src
         end
-        attr_reader :src
 
         def [](key)
           @src[key]
         end
+
+        def to_hash
+          @src
+        end
       end
     end
   end