Add consistent hash method to Rackstash::Fields::AbstractCollection

This follows the rule that two objects which are eql? also have the same
hash value. It is also required to ensure that we can use collections in
sets, hashes and arrays where the hash-equality is checked for certain
operations.

lib/rackstash/fields/abstract_collection.rb

@@ -22,6 +22,16 @@ module Rackstash
       end
       alias eql? ==
 
+      # Compute a hash-code for this collection.
+      #
+      # Two collections with the same raw content will have the same hash code
+      # (and will compare using {#eql?}).
+      #
+      # @return [Integer] the hash ID of `self`
+      def hash
+        [self.class, raw].hash
+      end
+
       # Show a human-readable representation of `self`. To get a machine-
       # readable "export" of the contained data, use {#as_json} or one of its
       # aliases instead.

spec/rackstash/fields/abstract_collection_spec.rb

@@ -91,6 +91,18 @@ describe Rackstash::Fields::AbstractCollection do
     end
   end
 
+  describe '#hash' do
+    it 'returns the same hash for the same raw content' do
+      collection.send(:raw=, [123, 'foo'])
+
+      collection2 = Rackstash::Fields::AbstractCollection.new
+      collection2.send(:raw=, [123, 'foo'])
+
+      expect(collection.send(:raw)).not_to equal collection2.send(:raw)
+      expect(collection.hash).to eql collection2.hash
+    end
+  end
+
   describe '#raw' do
     it 'is a protected accessor' do
       expect { collection.raw = nil }.to raise_error NoMethodError