Set Encoder helper methods as private

That way, they don't pollute the public interface of the encoders
including the helpers modules.

lib/rackstash/encoders/helpers/message.rb

@@ -10,6 +10,8 @@ module Rackstash
       # Some useful helper methods for {Encoders} which help in normalizing and
       # handling the message list in the event Hash.
       module Message
+        private
+
         # Normalize the `"message"` field of the given log event Hash.
         #
         # While the filters still had access to the array of {Message} objects
@@ -24,7 +26,7 @@ module Rackstash
         # @param event [Hash] a log event Hash
         # @return [Hash] the given event with the `"message"` key set as a
         #   single string.
-        def normalize_message(event)
+        def normalize_message(event) #:doc:
           event[FIELD_MESSAGE] =
             case event[FIELD_MESSAGE]
             when Array

lib/rackstash/encoders/helpers/timestamp.rb

@@ -14,6 +14,8 @@ module Rackstash
       # handling timestamps in the event Hash, especially the {FIELD_TIMESTAMP}
       # field.
       module Timestamp
+        private
+
         # Normalize the `"@timestamp"` field of the given log event Hash.
         # Before any filters, only the `"@timestamp"` fueld contains a `Time`
         # object denoting the timestamp of the log event. To represent this
@@ -25,7 +27,7 @@ module Rackstash
         #   hash. By default, we use the `"@timestamp"` field.
         # @return [Hash] the given event with the `field` key set as an ISO 8601
         #   formatted time string.
-        def normalize_timestamp(event, field: FIELD_TIMESTAMP)
+        def normalize_timestamp(event, field: FIELD_TIMESTAMP) #:doc:
           time = event[field]
 
           if time.is_a?(Time) || time.is_a?(DateTime)

spec/rackstash/encoders/helpers/message_spec.rb

@@ -9,7 +9,15 @@ require 'spec_helper'
 require 'rackstash/encoders/helpers/message'
 
 describe Rackstash::Encoders::Helpers::Message do
-  let(:helper) { Object.new.extend(described_class) }
+  let(:helper) {
+    helper = Object.new.extend(described_class)
+    described_class.private_instance_methods(false).each do |method|
+      helper.define_singleton_method(method) do |*args|
+        super(*args)
+      end
+    end
+    helper
+  }
   let(:event) { {} }
 
   describe '#normalize_message' do

spec/rackstash/encoders/helpers/timestamp_spec.rb

@@ -9,7 +9,15 @@ require 'spec_helper'
 require 'rackstash/encoders/helpers/timestamp'
 
 describe Rackstash::Encoders::Helpers::Timestamp do
-  let(:helper) { Object.new.extend(described_class) }
+  let(:helper) {
+    helper = Object.new.extend(described_class)
+    described_class.private_instance_methods(false).each do |method|
+      helper.define_singleton_method(method) do |*args|
+        super(*args)
+      end
+    end
+    helper
+  }
   let(:event) { {} }
 
   describe '#normalize_timestamp' do