Trace merging code is overzealous

[cpsievert]

In #335, I fixed #250 by not merging traces (with identical x/y/axis/yaxis) if the trace type is bar. However, this solution is not general enough, since this causes problems for other traces as well. For example,

library(plotly)
m <- ggplot(faithful, aes(x = eruptions, y = waiting)) +
 geom_point() +
 xlim(0.5, 6) +
 ylim(40, 110)
g <- m + geom_density2d()
ggplotly(g)

Error in if (is.character(new.mode) && !is.na(new.mode)) { : 
  missing value where TRUE/FALSE needed