Batary

Confirmed for 14w20b. But the testfor command works for me.

I got this problem too. Actually, this happened one time in two for me :
The first time I tried, it works. Then, with the exact same process, it didn't.
On the third try it worked, and so on. I've done this almost 20 times with the same result, alterning working and not working...
The dispenser tries to equip the coloured armor but it seems that the game keeps in memory your previously equipped colored armor. So if you only drop your armor, which i did and i think you do, the game will consider you as 'still equipped'. That's where i think the bug is. In any case, you have to clear the player's inventory before dispensing the armor :

/clear @p leather_helmet (or chestplate/leggings/boots)

or even

/clear @p

Thanks a lot !
I didn't know the "DropChances" tag can be set to a higher value than 1 : for me it was linked to a percentage. I still think this is a small bug, but thanks to you that's not really important now.
This topic can be set to resolved.

Confirmed. I used this command :
give @p writable_book 1 0 {display:{Name:"The Book"}} .

You have forgotten the "Count" value :
/summon Item ~ ~ ~3 {Item:{id:minecraft:diamond,Count:1}}