Allow to define a suitable default encoder per adapter

The global default (unless overwritten by an adapter class) is to use
the JSON encoder since it's the most versatile and flexible option for a
logger today.

lib/rackstash/adapters/adapter.rb

@@ -41,6 +41,15 @@ module Rackstash
       def initialize(*)
       end
 
+      # Return a new Encoder instance which can be used with the concrete adapter
+      # If no explicit encoder is defined in a {Flow}, this encoder will be used
+      # there
+      #
+      # @return [#call] an encoder
+      def default_encoder
+        Rackstash::Encoders::JSON.new
+      end
+
       # Close the underlying log device if supported by it.
       #
       # This method needs to be overwritten in child classes. By default, this

spec/rackstash/adapters/adapter_spec.rb

@@ -18,6 +18,12 @@ describe Rackstash::Adapters::Adapter do
     end
   end
 
+  describe '.default_encoder' do
+    it 'returns an encoder' do
+      expect(adapter.default_encoder).to respond_to(:encode)
+    end
+  end
+
   describe '#close' do
     it 'does nothing' do
       expect(adapter.close).to be nil