Boolean attributes were parsed the same way as string attributes,
so a value of { "bool_attr": "true" } would be parsed correctly, but
{ "bool_attr": true } (without quotes) was parsed as false.
Fixes FS#3284
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 7f676b5ed6a2bcd6786a0fcb6a6db3ddfeedf795)