Your charger specifications (0.9a, 14.7v) seem to be within the datasheets' specifications for charging.
As Retched suggested, you may have connected the charger to the battery improperly (reversed connections) or the battery or the charger could have been defective.
Before attempting to charge another battery, the charger performance should be verified.
Most "good" battery testers will apply a load to the battery under test.
This is how the "health" of batteries are usually tested.
For instance, a dead battery, left sitting, may read 12v with a standard multi-meter. But as soon as a load is connected, the voltage can drop to 10v or lower.
The automotive and deep cycle battery tester I use has a heating element with the voltage meter, allowing the the battery to be under load when tested.
If your fully charged battery is reading 12v, it is likely dead.. it should read ~13v.
How long was the battery left connected to the charger before it 'melted'? Overcharging can do this. Charging a battery with a shorted cell can also cause this to happen, and in that case, even using a proper duration of charging will not prevent it.
It's possible the battery may have had an intermittent bad cell as your charger voltage readings sound right. The reason it isn't drawing the full 0.9A would be due to a defective battery or one that was already almost fully charged.
It is normal for batteries to not use all the current available. Just because the label on the charger says it will provide .9 amps does not mean every battery will be willing to accept that.
as for the melted battery, I think the battery was bad before you connected it for a charge. Your measurements on the charger indicate that it is still working correctly. That means the melted battery was sick.