Fix enum hash equality

Protobuf::Enum delegates methods to Fixnum, which has a custom hash
equality method (`eql?`). This causes enum values to be equivalent when
using `==`, `===`, `equals?`, but not `eql?`**:

2.3.7 :002 > Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO)
 => false

However, they are equilvalent to their tag value:

2.3.7 :002 > Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO.tag)
 => true

Use the hash equality implementation from Object#eql?, which is
equivalent to == instead.

**The implementation changed in Ruby 2.5, so this only affects Ruby
versions less than v2.5.

Author: liveh2o

lib/protobuf/enum.rb

@@ -271,6 +271,25 @@ def class
       tag.class
     end
 
+    # Protobuf::Enum delegates methods to Fixnum, which has a custom hash equality method (`eql?`)
+    # This causes enum values to be equivalent when using `==`, `===`, `equals?`, but not `eql?`**:
+    #
+    #   2.3.7 :002 > ::Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO)
+    #    => false
+    #
+    # However, they are equilvalent to their tag value:
+    #
+    #   2.3.7 :002 > ::Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO.tag)
+    #    => true
+    #
+    # **The implementation changed in Ruby 2.5, so this only affects Ruby versions less than v2.4.
+    #
+    # Use the hash equality implementation from Object#eql?, which is equivalent to == instead.
+    #
+    def eql?(other)
+      self == other
+    end
+
     def inspect
       "\#<Protobuf::Enum(#{parent_class})::#{name}=#{tag}>"
     end

spec/lib/protobuf/enum_spec.rb

@@ -268,6 +268,20 @@
     specify { subject.try { |yielded| expect(yielded).to eq(subject) } }
   end
 
+  describe '#eql?' do
+    it "is equal to itself" do
+      expect(::Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO)).to be(true)
+    end
+
+    it "is equal to it's tag" do
+      expect(::Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO.tag)).to be(true)
+    end
+
+    it "is not equal to it's name" do
+      expect(::Test::EnumTestType::ZERO.eql?(::Test::EnumTestType::ZERO.name)).to be(false)
+    end
+  end
+
   context 'when coercing from enum' do
     subject { Test::StatusType::PENDING }
     it { is_expected.to eq(0) }