Add Rackstash::Fields::Hash#key? predicate

lib/rackstash/fields/hash.rb

@@ -76,6 +76,24 @@ module Rackstash
         @raw.empty?
       end
 
+      # Returns true if the given key is present in `self`.
+      #
+      #     h = Rackstash::Fields::Hash.new
+      #     h.merge!({ "a" => 100, "b" => 200 })
+      #
+      #     h.key?("a")   #=> true
+      #     h.key?("z")   #=> false
+      #
+      # @param key [#to_s] the field name. The key will be converted to an
+      #   UTF-8 string before being checked.
+      # @return [Boolean] `true` if the normalized key is present in `self`
+      def key?(key)
+        @raw.key? utf8_encode(key)
+      end
+      alias has_key? key?
+      alias include? key?
+      alias member? key?
+
       # @return [::Array] a new array populated with the keys from this hash.
       # @see #values
       def keys

spec/rackstash/fields/hash_spec.rb

@@ -179,6 +179,45 @@ describe Rackstash::Fields::Hash do
     end
   end
 
+  describe '#key?' do
+    it 'checks whether a key was set' do
+      hash['hello'] = 'World'
+      expect(hash.key?('hello')).to be true
+      expect(hash.key?('Hello')).to be false
+      expect(hash.key?('goodbye')).to be false
+    end
+
+    it 'checks keys with stringified names' do
+      hash['hello'] = 'World'
+      expect(hash.key?('hello')).to be true
+      expect(hash.key?(:hello)).to be true
+    end
+
+    it 'can use the alias #has_key?' do
+      hash['hello'] = 'World'
+      expect(hash.has_key?('hello')).to be true
+      expect(hash.has_key?('goodbye')).to be false
+
+      # We can also use the rspec matcher
+      expect(hash).to have_key 'hello'
+    end
+
+    it 'can use the alias #include?' do
+      hash['hello'] = 'World'
+      expect(hash.include?('hello')).to be true
+      expect(hash.include?('goodbye')).to be false
+
+      # We can also use the rspec matcher
+      expect(hash).to include 'hello'
+    end
+
+    it 'can use the alias #member?' do
+      hash['hello'] = 'World'
+      expect(hash.member?('hello')).to be true
+      expect(hash.member?('goodbye')).to be false
+    end
+  end
+
   describe '#keys' do
     it 'returns an array of keys' do
       hash['foo'] = 'bar'