title: time zone not working when plotting tweets with rtweet's ts_plot()
body: <p>there is a closed issue on rtweet's GitHub saying that you could use tz in ts_plot <a href="https://github.com/ropensci/rtweet/issues/227" rel="nofollow noreferrer">https://github.com/ropensci/rtweet/issues/227</a></p>
  
  <pre><code>rt &lt;- search_tweets("rstats", n = 500)
  
  ## with default timezone (UTC)
  ts_plot(rt, "hours")
  
  ## with american central time zone
  ts_plot(rt, "hours", tz = "US/Central") 
  </code></pre>
  
  <p>but i'm trying this in my code and I always get UTM hours </p>
  
  <pre><code>ts_plot(tweets, "mins", tz = "America/Montevideo") +
    labs(x = NULL, y = NULL,
         title = "Frequency of tweets",
         subtitle = paste0(format(min(tweets$created_at), "%d/%m/%Y - %H:%M:%S", tz = "America/Montevideo"), " to ", format(max(tweets$created_at),"%d/%m/%Y - %H:%M:%S", tz = "America/Montevideo")),
         caption = "Data collected from Twitter's REST API via rtweet") 
  </code></pre>
  
  <p>tz is working fine for the subtitle, but no for the actual t_plot, any ideas if this should be working or not?</p>
  
  <p>I downloaded the package directly from GitHub with</p>
  
  <pre><code>## install dev version of rtweet from github
  remotes::install_github("ropensci/rtweet")
  library(rtweet)
  </code></pre>
  
  <p>Thanks</p>