fix missing write mode in file handler
fix missing write mode in file handler

pavel-paulau/perfrunner,PaintScratcher/perfrunner,vmx/perfrunner,couchbase/perfrunner,EricACooper/perfrunner,dkao-cb/perfrunner,pavel-paulau/perfrunner,dkao-cb/perfrunner,pavel-paulau/perfrunner,pavel-paulau/perfrunner,mikewied/perfrunner,couchbase/perfrunner,pavel-paulau/perfrunner,hsharsha/perfrunner,mikewied/perfrunner,thomas-couchbase/perfrunner,EricACooper/perfrunner,couchbase/perfrunner,couchbase/perfrunner,EricACooper/perfrunner,EricACooper/perfrunner,couchbase/perfrunner,couchbase/perfrunner,vmx/perfrunner,hsharsha/perfrunner,thomas-couchbase/perfrunner,PaintScratcher/perfrunner