title: tsibble::yearmonth does not convert properly the format "2016.11" and "2016.12"
body: <p>There's a small issue with <code>tsibble::yearmonth</code>. See this list of year - months values:</p>
  <pre><code>x &lt;- c(&quot;2016.09&quot;, &quot;2016.10&quot;, &quot;2016.11&quot;, &quot;2016.12&quot;)
  </code></pre>
  <p>When I call <code>tsibble::yearmonth</code> (<code>tsibble</code> version 1.0.0), I obtain this:</p>
  <pre><code>tsibble::yearmonth(x)
  &lt;yearmonth[4]&gt;
  [1] &quot;2016 Sep&quot; &quot;2016 Oct&quot; &quot;2016 Jan&quot; &quot;2016 Feb&quot;
  </code></pre>
  <p>The function has correctly transformed September and October, but makes an error with November and December.</p>
  <p>Workaround exists, e.g.:</p>
  <pre><code>tsibble::yearmonth(as.Date(paste0(x, &quot;.01&quot;), format = &quot;%Y.%m.%d&quot;))
  &lt;yearmonth[4]&gt;
  [1] &quot;2016 Sep&quot; &quot;2016 Oct&quot; &quot;2016 Nov&quot; &quot;2016 Dec&quot;
  </code></pre>
  <p>but it would be nice if <code>yearmonth()</code> recognizes the date properly.</p>