(maint) Resolve config object before manipulating it

The ConfigDocument API that preserves comments and formatting in HOCON
files does not automatically resolve substitutions. This makes it
impossible to manipulate settings that involve substitutions. This
commit adds a `resolve` call to the config object that is parsed and
provided for manipulation.

lib/puppet/provider/hocon_setting/ruby.rb

@@ -107,7 +107,7 @@ def conf_object
     if @conf_file.nil? && !File.exist?(file_path)
       File.new(file_path, 'w')
     end
-    Hocon::ConfigFactory.parse_file(file_path)
+    Hocon::ConfigFactory.parse_file(file_path).resolve
   end
 
   def remove_value(value_to_remove)

spec/unit/puppet/provider/conf_setting/ruby_spec.rb

@@ -839,4 +839,33 @@ def validate_file(expected_content, tmp = tmpfile)
       expect(provider.value[0]).to eql('a' => 'b')
     end
   end
+
+  context 'substitutions' do
+    let(:orig_content) do
+      <<-EOS
+test_key_1:
+  {
+    bar: barvalue
+  }
+sub_key: ${test_key_1.bar}
+      EOS
+    end
+
+    it 'can change the value of a setting with a substitution' do
+      resource = Puppet::Type::Hocon_setting.new(common_params.merge(
+                                                   setting: 'sub_key', ensure: 'present', value: 'newvalue', type: 'string'
+      ))
+      provider = described_class.new(resource)
+      expect(provider.exists?).to be true
+      provider.value = 'newvalue'
+      validate_file(<<-EOS
+test_key_1:
+  {
+    bar: barvalue
+  }
+sub_key: "newvalue"
+      EOS
+                   )
+    end
+  end
 end