fix: [API] Fixed output of the attribute histogram
fix: [API] Fixed output of the attribute histogram

- no more STIX-ish barf inducing numeric string keys for dictionaries

SteveClement/MISP,MISP/MISP,MISP/MISP,MISP/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,SteveClement/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP