title: cmsfn.inherit() seems to return wrong type
body: <p>In Magnolia 4.5.3, I'm trying to inherit a parent page's stage if none is defined for this.
  The Java class <code>TemplatingFunctions</code> (exposed as <code>cmsfn</code> in freemarker) provides a method <code>inherit(Node content)</code>, so I'm trying the following:</p>
  
  <pre><code>[#if component??]
      [@cms.component content=component /]
  [#else]
      [#assign inheritedContent = cmsfn.inherit(cmsfn.content(state.mainContent.@handle)) /]
      [@cms.component content=inheritedContent /]
  [/#if]
  </code></pre>
  
  <p>But then I get an error:</p>
  
  <pre><code>The 'content' parameter must be a ContentMapModel and is a StringModel.
  </code></pre>
  
  <p>(line 6 in my example)</p>
  
  <p>Why does this error occur, i.e. how could I resolve such an error?</p>