In MCPE Iron Golem is caled: irongolem
In 1.10.2 PC version it's called: VillagerGolem
In 1.11 PC snapshots it's called: villager_golem
This is true for other entities as well, commands also don't accept Cow when summoning a cow. Only the lower-case version will get accepted.
This may not be a bug, but it is an inconsistency.
Thanks for pointing this out. At the moment, commands are just being introduced, so these improvements are not yet being considered. Please restrict new tickets to bugs you have identified in existing features in pocket edition.