(FACT-601) Format plaintext output for non-string values

Without this commit, structured facts outputted as plaintext are
printed using `#to_s`, which is useless on ruby 1.8.7:

    $ facter partitions
    sda1mount/bootsize1024000uuid7de786a9-be75-4711-b782-fb26aec963d4sda2size975747072sda3size2047

This commit changes the outputting to format non-string values using
`#inspect` which produces much more usable values:

    $ facter partitions
    {"sda1"=>{"uuid"=>"7de786a9-be75-4711-b782-fb26aec963d4", "size"=>"1024000", "mount"=>"/boot"}, "sda2"=>{"size"=>"975747072"}, "sda3"=>{"size"=>"2047"}}