title: Warning in gam with release of R 3.6.1
body: <p>I now see a warning when running <code>gam()</code>. MWE below:</p>
  
  <pre><code>library(gam)
  data(Boston, package = 'MASS')
  model.gam &lt;- gam(formula = medv ~ crim + zn, data = Boston)
  </code></pre>
  
  <p>The warning I receive is:</p>
  
  <pre><code>Warning message:
  In model.matrix.default(mt, mf, contrasts) :
     non-list contrasts argument ignored
  </code></pre>
  
  <p>I think this may relate to a change in the <code>model.matrix()</code> function in the latest R release: <a href="https://cran.r-project.org/doc/manuals/r-release/NEWS.html" rel="nofollow noreferrer">https://cran.r-project.org/doc/manuals/r-release/NEWS.html</a>. I did not receive a warning earlier today when running R version 3.5.</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.6.1 (2019-07-05)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows &gt;= 8 x64 (build 9200)
  
  Matrix products: default
  
  locale:
  [1] LC_COLLATE=English_United States.1252 
  [2] LC_CTYPE=English_United States.1252   
  [3] LC_MONETARY=English_United States.1252
  [4] LC_NUMERIC=C                          
  [5] LC_TIME=English_United States.1252    
  
  attached base packages:
  [1] splines   stats     graphics  grDevices utils     datasets 
  [7] methods   base     
  
  other attached packages:
  [1] gam_1.16      foreach_1.4.4
  
  loaded via a namespace (and not attached):
  [1] compiler_3.6.1   tools_3.6.1      yaml_2.2.0      
  [4] codetools_0.2-16 iterators_1.0.10
  </code></pre>