fix output_value not referencing correct key

[senorsmile]

This patch fixes the output_value var referencing, which would produce an error like

Use of uninitialized value $output_​value in concatenation (.​) or string at 
/usr/lib/nagios/plugins/check_​json.​pl line 251.

The output_value var was attempting to reference a key like:

$json_response->{$key}
where
$key = "{foo}"

However, that would interpolate as:
$json_repsonse->{"{foo}"}
which obviously doesn't work.

[senorsmile]

fixes (at least partially) #23

[pmoranga]

Wont this return $label just when it exists on the root of the json ??