title: get_map not passing the API key (HTTP status was '403 Forbidden')
body: <p>
  I have been facing this issue in the <code>get_map()</code> function (<code>ggmap</code> library) in R. </p>
  
  <p>My code was running without the need to specify an API key (for <code>source = "google"</code>) for several months. However, the code stopped working a couple of weeks back. I understood that Google has made the API key mandatory (or maybe they allowed a certain no of calls without the api key which I exhausted). </p>
  
  <p>However, even after specifying the API key (obtained from Google Cloud Platform) my code continued behaving the same way. I even contacted Google Cloud Support, but they said there was nothing wrong with the API key per se and they were able to invoke the map at their end. </p>
  
  <p>I suspect the <code>get_map()</code> function is not passing the <code>api_key</code> while invoking the map from Google. Any pointers towards resolution would be appreciated. </p>
  
  <p>Below is the reproducible code (that is failing). </p>
  
  <pre><code>library(ggmap)
  
  lat &lt;- c(4,41)  # India lat boundaries
  lon &lt;- c(68,99) # India long boundaries
  center = c(mean(lat), mean(lon))
  
  map &lt;- get_map(location = c(lon = mean(lon), 
                              lat = mean(lat)),
                 api_key = &lt;my api key&gt;,
                 zoom = 6,
                 maptype = "terrain",
                 source = "google",
                 messaging = TRUE
  )
  </code></pre>
  
  <p>And below is the error message in R (note the API key is not getting passed)</p>
  
  <pre><code>trying URL 'http://maps.googleapis.com/maps/api/staticmap?center=22.5,83.5&amp;zoom=6&amp;size=640x640&amp;scale=2&amp;maptype=terrain&amp;language=en-EN&amp;sensor=false'
  Error in download.file(url, destfile = tmp, quiet = !messaging, mode = "wb") : 
    cannot open URL 'http://maps.googleapis.com/maps/api/staticmap?center=22.5,83.5&amp;zoom=6&amp;size=640x640&amp;scale=2&amp;maptype=terrain&amp;language=en-EN&amp;sensor=false'
  In addition: Warning message:
  In download.file(url, destfile = tmp, quiet = !messaging, mode = "wb") :
    cannot open URL 'http://maps.googleapis.com/maps/api/staticmap?center=22.5,83.5&amp;zoom=6&amp;size=640x640&amp;scale=2&amp;maptype=terrain&amp;language=en-EN&amp;sensor=false': HTTP status was '403 Forbidden'
  </code></pre>