plugins/sentry_mail: decode subject as utf8 to prevent UnicodeDecodeError
plugins/sentry_mail: decode subject as utf8 to prevent UnicodeDecodeError

Fixes GH-1937

jean/sentry,BuildingLink/sentry,alexm92/sentry,fotinakis/sentry,imankulov/sentry,fotinakis/sentry,zenefits/sentry,korealerts1/sentry,ifduyue/sentry,zenefits/sentry,nicholasserra/sentry,alexm92/sentry,JamesMura/sentry,Kryz/sentry,ngonzalvez/sentry,BayanGroup/sentry,Natim/sentry,daevaorn/sentry,Kryz/sentry,imankulov/sentry,looker/sentry,fotinakis/sentry,daevaorn/sentry,ifduyue/sentry,nicholasserra/sentry,fotinakis/sentry,nicholasserra/sentry,mvaled/sentry,korealerts1/sentry,alexm92/sentry,BayanGroup/sentry,BuildingLink/sentry,jean/sentry,zenefits/sentry,daevaorn/sentry,beeftornado/sentry,JamesMura/sentry,looker/sentry,beeftornado/sentry,BuildingLink/sentry,felixbuenemann/sentry,imankulov/sentry,mvaled/sentry,looker/sentry,Natim/sentry,ifduyue/sentry,daevaorn/sentry,mvaled/sentry,kevinlondon/sentry,felixbuenemann/sentry,JackDanger/sentry,BayanGroup/sentry,kevinlondon/sentry,gencer/sentry,looker/sentry,gencer/sentry,mitsuhiko/sentry,JamesMura/sentry,mvaled/sentry,zenefits/sentry,gencer/sentry,beeftornado/sentry,mvaled/sentry,ngonzalvez/sentry,mvaled/sentry,BuildingLink/sentry,looker/sentry,zenefits/sentry,mitsuhiko/sentry,jean/sentry,felixbuenemann/sentry,JackDanger/sentry,korealerts1/sentry,kevinlondon/sentry,JamesMura/sentry,jean/sentry,ifduyue/sentry,Kryz/sentry,BuildingLink/sentry,gencer/sentry,Natim/sentry,ngonzalvez/sentry,gencer/sentry,JamesMura/sentry,jean/sentry,JackDanger/sentry,ifduyue/sentry