How to replace space with comma in sed

test.txt :

Bugs Bunny
Daffy Duck

cat test.txt | sed -e 's/[ ]/,/g'

outputs:

Bugs, Bunny
Daffy, Duck

cat test.txt | sed -e 's/[ ]/,/g' | sed 's/$/,/'
will append comma to the end as well, so:

Bugs, Bunny,
Daffy, Duck,