title: Error with date using gtrendsR
body: <p>Why the code below works just fine:</p>
  
  <pre><code>library(gtrendsR)
  
  ch &lt;- gconnect("usr@gmail.com", "psw")
  
  query1 &lt;- gtrends(query = c("Ludwig von Mises", "John Maynard Keynes", "Karl Marx"),
                   geo = c("BR"),
                   start_date = "2016-01-01",
                   end_date = "2016-03-31")
  </code></pre>
  
  <p>But if I change the date to:</p>
  
  <pre><code>query2 &lt;- gtrends(query = c("Ludwig von Mises", "John Maynard Keynes", "Karl Marx"),
                   geo = c("BR"),
                   start_date = "2010-01-01",
                   end_date = "2016-03-31")
  </code></pre>
  
  <p>R trows an error:</p>
  
  <pre><code>Error in as.POSIXlt.character(x, tz, ...) : 
    character string is not in a standard unambiguous format
  </code></pre>