Add Rackstash::Fields::Array#size as an alias to length

lib/rackstash/fields/array.rb

@@ -126,6 +126,7 @@ module Rackstash
       def length
         @raw.length
       end
+      alias size length
 
       # Set Union -- Add value from `array` to `self` excluding any duplicates
       # and preserving the order from `self`.

spec/rackstash/fields/array_spec.rb

@@ -228,6 +228,12 @@ describe Rackstash::Fields::Array do
       array.clear
       expect(array.length).to eql 0
     end
+
+    it 'can use size as an alias' do
+      expect(array.size).to eql 0
+      array[0] = 'first'
+      expect(array.size).to eql 1
+    end
   end
 
   describe '#merge' do