Subassignment to properties changes mode

[hughjonesd]

E.g. this breaks:

ht <- hux(a = 1:2)
top_padding(ht)[1, 1] <- '6px'

top_padding(ht) # shows that default 4 has become character '4'
print_html(ht)    # and character 4 is interpreted as its own unit.

[hughjonesd]

It is hard to warn about this because before e.g. top_padding<- is called, [<- has been called on top_padding(ht) to create the full matrix of values. Hence the mode has already changed to something new before you can detect it.