Messages fields should be able to be set with to_proto

A prior commit introduced a bug so that you would get `Failure/Error: fail TypeError, "Unacceptable value #{value} for field #{field.name} of type #{field.type_class}"` when trying to set a message field with a class that defined a #to_proto method.

Author: embark

lib/protobuf/field/message_field.rb

@@ -9,7 +9,7 @@ class MessageField < BaseField
       #
 
       def acceptable?(val)
-        val.is_a?(type_class) || val.respond_to?(:to_hash)
+        val.is_a?(type_class) || val.respond_to?(:to_hash) || val.respond_to?(:to_proto)
       end
 
       def decode(bytes)
@@ -35,8 +35,8 @@ def coerce!(value)
           nil
         elsif value.is_a?(type_class)
           value
-        elsif value.respond_to?(:to_proto)
-          value.to_proto
+        elsif value.respond_to?(:to_proto) && (proto_value = value.to_proto).is_a?(type_class)
+          proto_value
         elsif value.respond_to?(:to_hash)
           type_class.new(value.to_hash)
         else

spec/lib/protobuf/field/message_field_spec.rb

@@ -0,0 +1,68 @@
+require 'spec_helper'
+
+RSpec.describe Protobuf::Field::MessageField do
+  class MessageField < ::Protobuf::Message
+    optional :int32, :field, 1
+  end
+
+  class SomeMessage < ::Protobuf::Message
+    optional MessageField, :message_field, 1
+  end
+
+  class ToProtoMessageField
+    def to_proto
+      MessageField.new(:field => 42)
+    end
+  end
+
+  class ToProtoIsForWrongMessage
+    def to_proto
+      SomeMessage.new()
+    end
+  end
+
+  class ToHashMessageField
+    def to_hash
+      {:field => 989}
+    end
+  end
+
+  let(:instance) { SomeMessage.new }
+
+  describe 'setting and getting field' do
+    context "when set with the message type" do
+      let(:value) { MessageField.new(:field => 34) }
+
+      it 'is readable as a message' do
+        instance.message_field = value
+        expect(instance.message_field).to eq(value)
+      end
+    end
+
+    context "when set with #to_proto" do
+      let(:value) { ToProtoMessageField.new }
+
+      it 'is readable as a message' do
+        instance.message_field = value
+        expect(instance.message_field).to eq(value.to_proto)
+      end
+    end
+
+    context "when set with #to_proto that returns the wrong message type" do
+      let(:value) { ToProtoIsForWrongMessage.new }
+
+      it 'fails' do
+        expect { instance.message_field = value }.to raise_error TypeError
+      end
+    end
+
+    context "when set with #to_hash" do
+      let(:value) { ToHashMessageField.new }
+
+      it 'is readable as a message' do
+        instance.message_field = value
+        expect(instance.message_field).to eq(MessageField.new(value.to_hash))
+      end
+    end
+  end
+end