Fix calculation of theme defaults

**Issue**
At plot time, the calculation of theme elements always fell back
to the default theme. As a consequence, the default theme would
leak into complete themes where some of the elements were
unspecified.

**Solution**
Be more specific when calculating the theme defaults.

**Clean Up**
`theme_void` was insufficiently specified. The solution exposed
missing elements that had to be specified.

Fixes tidyverse#2058
Fixes tidyverse#2079

Author: has2k1

R/theme-defaults.r

@@ -336,9 +336,12 @@ theme_void <- function(base_size = 11, base_family = "") {
                          ),
     axis.text =        element_blank(),
     axis.title =       element_blank(),
+    axis.ticks.length =  unit(0, "pt"),
+    legend.position =    "right",
     legend.text =        element_text(size = rel(0.8)),
     legend.title =       element_text(hjust = 0),
     strip.text =         element_text(size = rel(0.8)),
+    panel.ontop =        FALSE,
     plot.margin =        unit(c(0, 0, 0, 0), "lines"),
 
     complete = TRUE

R/theme.r

@@ -380,7 +380,13 @@ theme <- function(line,
 
 # Combine plot defaults with current theme to get complete theme for a plot
 plot_theme <- function(x) {
-  defaults(x$theme, theme_get())
+  if (is.null(attr(x$theme, "complete"))) {
+    theme_get()
+  } else if (attr(x$theme, "complete")) {
+    x$theme
+  } else {
+    defaults(x$theme, theme_get())
+  }
 }
 
 #' Modify properties of an element in a theme object