title: No non-missing arguments warning when using min or max in reshape2
body: <p>I get the following warning when I use min or max in the dcast function from the reshape2 package. What is it telling me? I can't find anything that explains the warning message and I'm a bit confused about why I get it when I use max but not when I use mean or other aggregate functions.</p>
  
  <blockquote>
    <p>Warning message:<br>In .fun(.value[0], ...) : no non-missing arguments to min; returning Inf</p>
  </blockquote>
  
  <p>Here's a reproducible example:</p>
  
  <pre><code>data(iris)
  
  library(reshape2)
  
  molten.iris &lt;- melt(iris,id.var="Species")
  summary(molten.iris)
  str(molten.iris)
  #------------------------------------------------------------
  # Both return warning:
  dcast(data=molten.iris,Species~variable,value.var="value",fun.aggregate=min)
  dcast(data=molten.iris,Species~variable,value.var="value",fun.aggregate=max)
  
  # Length looks fine though
  dcast(data=molten.iris,Species~variable,value.var="value",fun.aggregate=length)
  
  #------------------------------------------------------------
  # No warning messages here:
  aggregate(value ~ Species + variable, FUN=min, data=molten.iris)
  aggregate(value ~ Species + variable, FUN=max, data=molten.iris)
  #------------------------------------------------------------
  # Or here:
  library(plyr)
  
  ddply(molten.iris,c("Species","variable"),function(df){
    data.frame(
      "min"=min(df$value),
      "max"=max(df$value)
      )
  })
  #------------------------------------------------------------
  </code></pre>