I put the command

/summon Zombie ~ ~5 ~ {Equipment:[{},{id:"minecraft:chainmail_helmet"}]}

into a command block, and it gives me a zombie with diamond boots. I tried this with all other armor types and it always gives me a zombie with diamond boots. If I remove the {} in the beginning of the equipment tag, nothing works. If I were to put anything besides boots AND boots on, it shows a zombie with pants and boots. If I put another on (that isn't already on) It shows a zombie with a chestplate, boots, and pants. Also, If I JUST put a helmet on, the zombie still burns, and the boots (the helmet) act like boots.