fix: replace Fixnum with Integer

closes #996

lib/postal/message_db/database.rb

@@ -348,7 +348,7 @@ module Postal
 
       def hash_to_sql(hash, joiner = ', ')
         hash.map do |key, value|
-          if value.is_a?(Array) && value.all? { |v| v.is_a?(Fixnum) }
+          if value.is_a?(Array) && value.all? { |v| v.is_a?(Integer) }
             "`#{key}` IN (#{value.join(', ')})"
           elsif value.is_a?(Array)
             escaped_values = value.map { |v| escape(v) }.join(', ')

lib/postal/message_db/message.rb

@@ -6,7 +6,7 @@ module Postal
       end
 
       def self.find_one(database, query)
-        query = {:id => query.to_i} if query.is_a?(Fixnum)
+        query = {:id => query.to_i} if query.is_a?(Integer)
         if message = database.select('messages', :where => query, :limit => 1).first
           Message.new(database, message)
         else