title: Accessing a data frame by rowname is giving me NA rather than the value I expect
body: <p>Here is an example.  normal is a dataframe with rownames equal to the month and day of an unspecified year (like "-01-01" for Jan 1). The stats listed are over some range of years.</p>
  
  <pre><code>head(normal)
  
  Source: local data frame [6 x 7]
  
        md    meanA      sdA  meanMax    sdMax  meanMin    sdMin
  1 -01-01 40.20556 1.424404 43.20185 1.792473 37.20926 1.575386
  2 -01-02 40.33056 1.473267 43.16852 2.527014 37.49259 1.480592
  3 -01-03 40.29630 1.645513 43.15926 2.347826 37.43333 1.572662
  4 -01-04 40.17222 1.635409 42.85000 2.502623 37.49444 1.512707
  5 -01-05 40.33796 1.681560 43.22593 2.486444 37.45000 1.511158
  6 -01-06 40.22685 1.767241 42.98704 2.380686 37.46667 1.591779
  
  md
  #[1] "-01-01"
  
  normal[1,"meanA"]  
  #Source: local data frame [1 x 1]
  
  #      meanA
  # 1 40.20556
  
  normal[md,"meanA"]    
  #Source: local data frame [1 x 1]
  
  #   meanA
  # 1    NA
  </code></pre>
  
  <p>Why won't R take the value of a rowname here as they are all unique?</p>