title: roll data.table with rollends
body: <p>I'm having trouble getting <code>roll=-Inf</code> to work when defining <code>rollends=FALSE</code>.  When <code>rollends</code> is not set or set to <code>TRUE</code>, I see the expected result.  I would appreciate any suggestions.</p>
  
  <pre><code>library(data.table)
  
  dt1 = data.table(Date=seq(from=as.Date("2013-01-03"),
                            to=as.Date("2013-06-27"), by="1 week"),
                   key="Date")[, ind:=.I]
  dt2 = data.table(Date=seq(from=as.Date("2013-01-01"),
                            to=as.Date("2013-06-30"), by="1 day"),
                   key="Date")
  </code></pre>
  
  <p>I would expect to see <code>2013-01-05</code> and <code>2013-06-26</code> populated in the output below.   </p>
  
  <pre><code>dt1[dt2, roll=-Inf, rollends=FALSE]
             Date ind
    1: 2013-01-01  NA
    2: 2013-01-02  NA
    3: 2013-01-03   1
    4: 2013-01-04   2
    5: 2013-01-05  NA
   ---               
  177: 2013-06-26  NA
  178: 2013-06-27  26
  179: 2013-06-28  NA
  180: 2013-06-29  NA
  181: 2013-06-30  NA
  </code></pre>
  
  <p>This is working as expected, the <code>ind</code> data is rolled forward but not outside of the endpoints defined in the <code>dt1</code> <code>data.table</code>.</p>
  
  <pre><code>dt1[dt2, roll=-Inf]
             Date ind
    1: 2013-01-01   1
    2: 2013-01-02   1
    3: 2013-01-03   1
    4: 2013-01-04   2
    5: 2013-01-05   2
   ---               
  177: 2013-06-26  26
  178: 2013-06-27  26
  179: 2013-06-28  NA
  180: 2013-06-29  NA
  181: 2013-06-30  NA
  </code></pre>