set default val for asp.ratio arg in stat_delvor_summary()

[ewenme]

stat_delvor_summary() is hitting an error due to a recursive default argument:

library(ggforce)
#> Loading required package: ggplot2
ggplot(iris, aes(Sepal.Length, Sepal.Width, group = -1L)) +
  stat_delvor_summary()
#> Error in layer(data = data, mapping = mapping, stat = StatDelvorSummary, : promise already under evaluation: recursive default argument reference or earlier problems?

^{Created on 2019-10-26 by the reprex package (v0.3.0)}

The offending argument is asp.ratio. This PR sets asp.ratio to 1 as default (as is the case in other voronoi/delaunay funcs in the pkg).