code: public void setContentType(String contentType) {
          if (contentType == null) {
              _properties.remove(AMQP_PROP_CONTENT_TYPE);
              return;
          }
          
          _properties.put(AMQP_PROP_CONTENT_TYPE, contentType);
      }
docs: Sets the value of "contentType" property. If a null value is passed in, it
  indicates that the property is not set.
  
  @param  contentType    value of "contentType" property