title: ggsn scale bar issue
body: <p>I'm hoping to get some help with the ggsn scale bar for my ggmap figure. 
  I am having trouble resizing the scale bar. The text is overlaying the line and I can't resize it into the filled in bar. </p>
  
  <p>I've tried adjusting the height,st.size, and st.dist  parameters, but these don't seem to change the scale bar. </p>
  
  <p>I'd appreciate any insights. Could this be due to the extreme zoomed in scale of my ggmap?</p>
  
  <p><a href="https://i.stack.imgur.com/X3P5g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/X3P5g.png" alt="Current map version"></a></p>
  
  <pre><code>MAD_map &lt;-  ggmap(get_map(location=c(-89.410087,43.074243), zoom=14, scale=4, source = "stamen", maptype = "terrain")) +theme(axis.title.x=element_blank(),            axis.text.x=element_blank(),axis.ticks.x=element_blank(),axis.title.y=element_blank(),
                     axis.text.y=element_blank(),axis.ticks.y=element_blank() ) +
  
    ggsn::scalebar(x.min = -89.387949, x.max = -89.387952, 
                   y.min =43.090330, y.max = 43.090340,location="topright", 
                   dist = .5, dd2km = TRUE, model = "GRS80", height =.2, st.size = 4, st.dist = .1
  
                   )
  </code></pre>