title: Issue Merging Dataframes: Warning joining factor and character vector
body: <p>I have two data frames that look like so</p>
  
  <pre><code>&gt; df1
  county   state   code
  ANDERSON Texas   1      
  ANDREWS  Texas   2
  ANGELINA Texas   3
  ....
  
  &gt; df2
  county   state   citations  year
  ANDERSON Texas   124        2011
  ANDREWS  Texas   32         2011
  ANGELINA Texas   491        2011
  ....
  </code></pre>
  
  <p>I have tried to merge the two of these a few different ways:</p>
  
  <pre><code>merge &lt;- full_join(df1, df2, by = c("county", "state"))
  merge &lt;- merge(df1, df2, by = c("county", "state"))
  </code></pre>
  
  <p>In both cases, I receive the following warning: </p>
  
  <pre><code>Warning message:
  Column `county` joining factor and character vector, coercing into
  character vector
  </code></pre>
  
  <p>The resulting data frame does not have any data for df2, even after coercing the factor into a character. I tried it again after turning the county column into a character in both data frames and still have issues.</p>
  
  <p>Here are the heads of the two data frames I am attempting to merge:</p>
  
  <pre><code>&gt; dput(head(data))
  structure(list(year = c(2011L, 2011L, 2011L, 2011L, 2011L, 2011L
  ), month = c(1L, 1L, 1L, 1L, 1L, 1L), county = c("ANDERSON COUNTY", 
  "ANGELINA COUNTY", "ARANSAS COUNTY", "ATASCOSA COUNTY", "BASTROP COUNTY", 
  "BELL COUNTY"), state = structure(c(2L, 2L, 2L, 2L, 2L, 2L), .Label = c("Montana", 
  "Texas"), class = "factor"), citations = c(218L, 422L, 55L, 472L, 
  745L, 1403L), warnings = c(521L, 711L, 124L, 1173L, 819L, 2242L
  ), population = c(56760L, 82812L, 24721L, 43589L, 72248L, 276975L
  ), d_revenue = c(-736L, -6723L, 1134L, 71L, 2308L, 852L), crashes = c(73L, 
  133L, 18L, 71L, 95L, 422L), density = c(55, 108.8, 91.9, 36.8, 
  83.5, 295.2), unemp_rate = c(8, 8.3, 9.6, 8.5, 8.5, 8), stops = 
  c(739L, 1133L, 179L, 1645L, 1564L, 3645L), stops_per_cap = c(0.013019732, 
  0.013681592, 0.007240807, 0.037738879, 0.021647658, 0.013160032
  ), crashes_per_cap = c(0.001286117, 0.001606047, 0.000728126, 
  0.001628851, 0.001314915, 0.001523603)), .Names = c("year", "month", 
  "county", "state", "citations", "warnings", "population", "d_revenue", 
  "crashes", "density", "unemp_rate", "stops", "stops_per_cap", 
  "crashes_per_cap"), row.names = c(NA, 6L), class = "data.frame")
  
  &gt; dput(head(codes))
  structure(list(county = c("ANDERSON  COUNTY ", "ANDREWS  COUNTY ", 
  "ANGELINA  COUNTY ", "ARANSAS  COUNTY ", "ARCHER  COUNTY ", "ARMSTRONG  COUNTY "
  ), state = structure(c(2L, 2L, 2L, 2L, 2L, 2L), .Label = c("Montana", 
  "Texas"), class = "factor"), code = 1:6), .Names = c("county", 
  "state", "code"), row.names = c(NA, 6L), class = "data.frame")
  </code></pre>