Heatmap z event data should derive from _z calc attribute

[cpsievert]

Closes #1141

TESTING NOTES

Run the following and make sure that by clicking on each cell of the heatmap, it prints the correct information (gee, it sure would be nice to have rstudio/shinytest#193 to unit test this!)

# devtools::install_github("ropensci/plotly#1393")
library(shiny)
library(plotly)

ui <- fluidPage(
        
        fluidRow(column(6, h4("Heatmap based on data.frame()"),
                        plotlyOutput("plot1"),
                        h4("Selected point in heatmap"),
                        verbatimTextOutput("selected1")
                ), 
                column(6, 
                        h4("Heatmap based on matrix()"),
                        plotlyOutput("plot2"),
                        h4("Selected point in heatmap"),
                        verbatimTextOutput("selected2")
                )
        )
)

server <- function(input, output) {
    
    randomValues <- rnorm(9)
    
    # Heatmap based on data.frame(): not working correctly, try to select x = b; y = 1
    output$plot1 <- renderPlotly({
                
                set.seed(1)
                myData <- data.frame(x = rep(letters[1:3], times = 3), 
                        y = rep(as.character(1:3), each = 3), value = randomValues)
                myData$value[1] <- NA
                
                plot_ly(
                        x = ~x, y = ~y, z = ~value, data = myData,
                        type = "heatmap", source = "plot1"
                )
                
            })
    
    
    output$selected1 <- renderPrint(
            event_data("plotly_click", source = "plot1")
    )
    
    
    # Heatmap based on matrix()
    output$plot2 <- renderPlotly({
                
                m <- matrix(randomValues, nrow = 3, ncol = 3, byrow = TRUE)
                m[1,1] <- NA
                
                plot_ly(
                        x = c("a", "b", "c"), y = as.character(1:3),
                        z = m, type = "heatmap", source = "plot2"
                )
                
                
            })
    
    
    output$selected2 <- renderPrint(
            event_data("plotly_click", source = "plot2")
    )
}

shinyApp(ui = ui, server = server)