title: Seedstack - MQTT Add-on problem adding user & pasword
body: <p>I try to connect to MQTT server and I have some problems adding the password:</p>
  
  <p>The configuration file is:</p>
  
  <pre><code>mqtt:
    clients:
      client-ptl-mqtt: 
        serverUri: tcp://rdabeg01.inetpsa.com:1883
        clientId: LOCAL_PTL_SIMUL_MQTT
        connection:
          userName: xxxxxx
          password: xxxxxxx
        reconnectionMode: ALWAYS
  </code></pre>
  
  <p>And the error when i try to start the application is:</p>
  
  <blockquote>
    <p>org.seedstack.coffig.internal.ConfigurationException: [CONFIGURATION]
    Illegal conversion</p>
    
    <p>Description
    ----------- Cannot convert to char(char.java:1): 'xxxxxxx'.</p>
    
    <h2>Stacktrace</h2>
  
  <pre><code>    at org.seedstack.coffig.internal.ConfigurationException.createNew(ConfigurationException.java:30)
      at org.seedstack.coffig.mapper.ValueMapper.charOf(ValueMapper.java:63)
      at org.seedstack.coffig.mapper.ValueMapper.map(ValueMapper.java:51)
      at org.seedstack.coffig.mapper.CompositeMapper.map(CompositeMapper.java:50)
      at org.seedstack.coffig.mapper.EvaluatingMapper.map(EvaluatingMapper.java:65)`
  </code></pre>
  </blockquote>