Allow Nil in Metadata

Issue: #343

Author: paneq

ruby_event_store/lib/ruby_event_store/metadata.rb

@@ -43,7 +43,7 @@ def each(&block)
     private
 
     def allowed_types
-      [String, Integer, Float, Date, Time, TrueClass, FalseClass]
+      [String, Integer, Float, Date, Time, TrueClass, FalseClass, nil]
     end
 
     private_constant :SAFE_HASH_METHODS

ruby_event_store/spec/mappers/protobuf_spec.rb

@@ -176,6 +176,7 @@ module Mappers
         six: Time.utc(2018, 12, 13, 11 ),
         seven: true,
         eight: false,
+        nein: nil,
       } }
       let(:data) do
         ResTesting::OrderCreated.new(

ruby_event_store/spec/metadata_spec.rb

@@ -34,6 +34,9 @@ module RubyEventStore
       m[:key] = false
       expect(m[:key]).to eq(false)
 
+      m[:key] = nil
+      expect(m[:key]).to eq(nil)
+
       expect do
         m[:key] = Object.new
       end.to raise_error(ArgumentError)