cubicweb #2410462 incompatiblity with cwtags [validation pending]

in one place (formrenderers), we do:

w(''.join(data))

instead of:

w(''.join(unicode(x) for x in data))

prioritynormal
typebug
done in3.15.2
load0.100
load left0.000
closed by#594efcf51536 [formrenderers] if a tag object gets there, it blows (closes #2410462)