[JENKINS-16881] Missing XML Declaration in Node's config.xml

```
$ curl http://localhost:8080/computer/%28master%29/config.xml
<hudson>
  <disabledAdministrativeMonitors/>
  <version>1.0</version>
  <numExecutors>2</numExecutors>
  <mode>NORMAL</mode>
  <useSecurity>true</useSecurity>
  <authorizationStrategy class="hudson.security.AuthorizationStrategy$Unsecured"/>
  <securityRealm class="hudson.security.SecurityRealm$None"/>
  <projectNamingStrategy class="jenkins.model.ProjectNamingStrategy$DefaultProjectNamingStrategy"/>
  <workspaceDir>${ITEM_ROOTDIR}/workspace</workspaceDir>
  <buildsDir>${ITEM_ROOTDIR}/builds</buildsDir>
  <jdks/>
  <viewsTabBar class="hudson.views.DefaultViewsTabBar"/>
  <myViewsTabBar class="hudson.views.DefaultMyViewsTabBar"/>
  <clouds/>
  <slaves>
    <slave>
      <name>test</name>
      <description></description>
      <remoteFS></remoteFS>
      <numExecutors>1</numExecutors>
      <mode>NORMAL</mode>
      <retentionStrategy class="hudson.slaves.RetentionStrategy$Always"/>
      <launcher class="hudson.slaves.JNLPLauncher"/>
      <label></label>
      <nodeProperties/>
      <userId>anonymous</userId>
    </slave>
  </slaves>
  <scmCheckoutRetryCount>0</scmCheckoutRetryCount>
  <views>
    <hudson.model.AllView>
      <owner class="hudson" reference="../../.."/>
      <name>すべて</name>
      <filterExecutors>false</filterExecutors>
      <filterQueue>false</filterQueue>
      <properties class="hudson.model.View$PropertyList"/>
    </hudson.model.AllView>
  </views>
  <primaryView>すべて</primaryView>
  <slaveAgentPort>0</slaveAgentPort>
  <label></label>
  <nodeProperties/>
  <globalNodeProperties/>
</hudson>
```

This happens since https://github.com/jenkinsci/jenkins/commit/660c0f6e61b2616da93ffb82009d5e6ad9c29c23 is released.