Fix wrong tooltip when ggplotly with geom_line(group=)

[zeehio]

This test case shows a plot with two lines where the tooltip does not contain the right text on all the points

The group aesthetic in geom_line is not properly set in the tooltip rendering mismatched "id: Sample2" tooltips on Sample1 data points and viceversa.

  # Tooltips with y == 10 should belong to Sample2 in this example
  mydata <- data.frame(id = paste0("Sample", rep(1:2, times = 4)),
                       x = rep(1:4, each = 2),
                       y = rep(c(1, 10), times = 4),
                       stringsAsFactors = FALSE)
  #        id x  y
  #1 Sample1 1  1
  #2 Sample2 1 10
  #3 Sample1 2  1
  #4 Sample2 2 10
  #5 Sample1 3  1
  #6 Sample2 3 10
  #7 Sample1 4  1
  #8 Sample2 4 10
  gplt <- ggplot(mydata) + geom_line(aes(x=x, y = y, group = id))
  pltly <- plotly::ggplotly(gplt)
  pltly
  y_equal_ten <- grepl("y: 10", pltly$x$data[[1]]$text)
  sample_2 <- grepl("id: Sample2", pltly$x$data[[1]]$text)
  expect_that(all(y_equal_ten == sample_2))

The tooltip in the image should show "id: Sample2"

Checked against current master:

plotly        4.5.5.9000 2016-10-28 Github (ropensci/plotly@32020c2)

[zeehio]

Closes #774

[cpsievert]

Great work, thanks!