title: coerce_timestamps does not work in write_parquet in Apache arrow R package
body: <p>i am trying to truncate timestamps to milliseconds when writing a parquet file.</p>
  <p>with:</p>
  <pre><code>tutu &lt;- as.POSIXct(&quot;2020/06/03 18:00:00&quot;,tz = &quot;UTC&quot;)
  </code></pre>
  <p>if i do:</p>
  <pre><code>write_parquet(data.frame(tutu),&quot;~/Downloads/tutu.test.parquet&quot;)
  </code></pre>
  <p>i get 1591207200000000</p>
  <p>if i do:</p>
  <pre><code>write_parquet(data.frame(tutu),&quot;~/Downloads/tutu.test.parquet&quot;, coerce_timestamps = &quot;ms&quot;, allow_truncated_timestamps = TRUE)
  </code></pre>
  <p>i get the error message:</p>
  <pre><code>Error in parquet___ArrowWriterProperties___Builder__coerce_timestamps(unit) : 
    argument &quot;unit&quot; is missing, with no default
  </code></pre>
  <p>what am i doing wrong?
  thanks in advance.</p>