Fix e_legend such that icon argument can be used.

[yogat3ch]

R's partial matching of arguments made it such that a single value passed as argument icon to e_legend when multiple legend$data objects are present in the chart causes this error:

Error in echarts4r::e_legend(e, icon = "circle") : 
  invalid number of icons; 1 icons passed but 4 legend items.

This modifies e_legend to allow for the expected behavior which uses it as the icon argument in opts$legend effectively changing the icon for the entire legend.