Log Responses
Install the dump_io mod by running this as root:
a2enmod dump_io
Add these lines to the bottom of the /etc/apache2/apache2.conf file:
LogLevel dumpio:trace7
DumpIOInput On
DumpIOOutput On
Restart the apache service to enact the changes:
service apache2 restart
View response data with this command:
cat error.log | cut -f8- -d':' \
    | egrep -v ' [0-9]+ bytes$' \
    | grep -v '^$' | cut -c2- | sed 's/\\r\\n//'
Run as an infinite loop:
while true; do
    cat error.log | cut -f8- -d':' \
        | egrep -v ' [0-9]+ bytes$' \
        | grep -v '^$' | cut -c2- \
        | sed 's/\\r\\n//'; sleep 5; done
Resources: https://httpd.apache.org/docs/2.4/mod/mod_dumpio.html https://serverfault.com/questions/445720/i-cant-get-the-dump-io-module-working