title: Numeric error when attempting to use rbind
body: <p>I'm attempting to create an extra row at the bottom of the below df that creates a total for each column. This was done using <code>rbind(Men_15, colSums(Men_15[1:52,])</code>. However, given the first couple of rows are factor and not numeric, I get the error message, "'x' must be numeric". How can I get R to ignore the irrelevant columns and sum just the numeric columns?
  I also want to name the row "USA"</p>
  
  <pre><code>   structure(list(State = structure(1:52, .Label = c("Alabama", 
  "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", 
  "Delaware", "District of Columbia", "Florida", "Georgia", "Hawaii", 
  "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", 
  "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", 
  "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", 
  "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", 
  "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", 
  "Pennsylvania", "Puerto Rico", "Rhode Island", "South Carolina", 
  "South Dakota", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", 
  "Washington", "West Virginia", "Wisconsin", "Wyoming"), class = "factor"), 
      Men = c(2341093L, 384160L, 3299088L, 1451913L, 19087135L, 
      2648667L, 1751607L, 448413L, 306674L, 9600009L, 4883331L, 
      709871L, 810464L, 6316899L, 3235263L, 1534595L, 1439862L, 
      2164208L, 2261156L, 650081L, 2872643L, 3249650L, 4861973L, 
      2692166L, 1451723L, 2964003L, 510163L, 929606L, 1407735L, 
      653484L, 4343027L, 1032414L, 9541801L, 4795408L, 367963L, 
      5662893L, 1906944L, 1948453L, 6245344L, 1713860L, 510388L, 
      2322409L, 423477L, 3167756L, 13171316L, 1459229L, 308573L, 
      4060948L, 3487725L, 913631L, 2851385L, 295561L)), class = c("tbl_df", 
  "tbl", "data.frame"), row.names = c(NA, -52L))
  </code></pre>