title: R quantmod buildModel() does not fit model
body: <p>I can't ride of an error when fitting model with <code>quantmod</code>, <code>buildModel</code> seems to not work and so use of <code>tradeModel</code> rise error :</p>
  
  <pre><code>getSymbols('GS',src='yahoo')
  
  q.model = specifyModel(Next(OpCl(GS)) ~ Lag(OpHi(GS),0:3))
  
  buildModel(q.model, method='lm',training.per=c('2007-08-01','2007-09-30'))
  &gt; q.model # it seems that model is still 'None fitted' ?
  
  quantmod object:        Build date:   
  
  Model Specified: 
       Next(OpCl(GS)) ~ Lag(OpHi(GS), 0:3) 
  
  Model Target:  Next.OpCl.GS              Product:  GS 
  Model Inputs:   
  
  Fitted Model: 
  
          None Fitted
  
  tradeModel(q.model,plot.model=TRUE,trade.dates=c("2008-01-01","2008-12-31"))
  
  Error in UseMethod("predict") : 
    no applicable method for 'predict' applied to an object of class "NULL"
  </code></pre>