Add more bar tests

Just making a note so I don't forget -- add more tests for 71e29b4

# https://github.com/ropensci/plotly/issues/831
df <- data.frame(
  V1 = gl(6,20), V2 = gl(40,3)
)
p <- ggplot(df, aes(x = V1, fill = V2)) + geom_bar(color="white")
ggplotly(p)


# https://github.com/ropensci/plotly/issues/560
d <- data.frame(
  x = LETTERS[1:10], 
  y = -5:4, 
  g = rep(c('A','B'), each = 5)
)

p <- ggplot(data = d, aes(x = x, y = y, fill = g)) +
  geom_bar(stat='identity')
ggplotly(p)

# https://github.com/ropensci/plotly/issues/901
data_before <- data.frame(
  year = c(2014,2014,2014,2015,2015,2015,2016,2016,2016),
  pet = c("dog","cat","bird","dog","cat","bird","dog","cat","bird"),
  wt = c(56, 25, 26, 10, 19, 41, 16, 17, 13)
)
data_after <- data.frame(
  year = c(2014,2014,2014,2015,2015,2015,2016,2016,2016),
  pet = c("dog","cat","bird","dog","cat","bird","dog","cat","bird"),
  wt = c(49, 18, 19,  3, 12, 34,  9, 10,  6)
)
p <- ggplot() +
  geom_bar(data = data_before, aes(year, wt, fill = pet), 
           stat = "identity", position = "dodge", width = 0.9, alpha = 0.5) +
  geom_bar(data = data_after, aes(year, wt, fill = pet), 
           stat = "identity", position = "dodge", width = 0.9) +
  labs(x = "Year", y = "Weight") +
ggplotly(p)

# https://github.com/ropensci/plotly/issues/874
dd <- data.frame(x = rnorm(100,5,2))
dd2 <- data.frame(x = rnorm(100,5,2))
p3 <- ggplot() + 
  geom_histogram(data = dd,aes(x = x,y = ..count..)) + 
  geom_histogram(data = dd2,aes(x = x,y = -..count..),color = 'green')
ggplotly(p3)



# https://github.com/ropensci/plotly/issues/968

p <- ggplot(data=dat, aes(x=time, y=total_bill)) +
  geom_bar(stat="identity") + coord_flip()
ggplotly(p)

Non sono indicati né un file di test né un punto di ingresso. Inizia individuando i test ggplotly esistenti per i grafici a barre e gli istogrammi, quindi esamina gli esempi di riproduzione collegati alle issue 831, 560, 901, 874 e 968; il lavoro è completato quando ogni caso dispone di una copertura di regressione e il comportamento previsto del grafico è stato documentato.