Bug 1898652: Avoid sending pdf browser check in response payload

The download serving for pdf export also sends back the buffer info
for the chrome browser check and therefore errors out the json with
parse error

We can avoid this by clearing the buffer after making the pdfs

Change-Id: I74991a3f80198f4f15e547b7b12c8ee68fd4ec36
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>