No Changes to mob spawning happend in 1.8, im not sure why yours isnt working but this is what i can tell you. These are all the changes that did happen in 1.8
Added Granite, Andesite, and Diorite stone blocks, with smooth versions
Added Slime Block
Added Iron Trapdoor
Added Prismarine and Sea Lantern blocks
Added the Ocean Monument
Added Red Sandstone
Added Banners
Added Armor Stands
Added Coarse Dirt (dirt where grass won’t grow)
Added Guardian mobs, with item drops
Added Endermite mob
Added Rabbits, with item drops
Added Mutton and Cooked Mutton
Villagers will harvest crops and plant new ones
Mossy Cobblestone and Mossy Stone Bricks are now craftable
Chiseled Stone Bricks are now craftable
Doors and fences now come in all wood type variants
Sponge block has regained its water-absorbing ability and becomes wet
Added a spectator game mode (game mode 3)
Added one new achievement
Added “Customized” world type
Added hidden “Debug Mode” world type
Worlds can now have a world barrier
Added @e target selector for Command Blocks
Added /blockdata command
Added /clone command
Added /execute command
Added /fill command
Added /particle command
Added /testforblocks command
Added /title command
Added /trigger command
Added /worldborder command
Added /stats command
Containers can be locked in custom maps by using the “Lock” data tag
Added logAdminCommands, showDeathMessages, reducedDebugInfo, sendCommandFeedback, and randomTickSpeed game rules
Added three new statistics
Player skins can now have double layers across the whole model, and left/right arms/legs can be edited independently
Added a new player model with smaller arms, and a new player skin called Alex?
Added options for configuring what pieces of the skin that are visible
Blocks can now have custom visual variations in the resource packs
Minecraft Realms now has an activity chart, so you can see who has been online
Minecraft Realms now lets you upload your maps
Difficulty setting is saved per world, and can be locked if wanted
Enchanting has been redone, now costs lapis lazuli in addition to enchantment levels
Villager trading has been rebalanced
Anvil repairing has been rebalanced
Considerable faster client-side performance
Max render distance has been increased to 32 chunks (512 blocks)
Adventure mode now prevents you from destroying blocks, unless your items have the CanDestroy data tag
Resource packs can now also define the shape of blocks and items, and not just their textures
Scoreboards have been given a lot of new features
Tweaked the F3 debug screen
Block ID numbers (such as 1 for stone), are being replaced by ID names (such as minecraft:stone)
Server list has been improved
A few minor changes to village and temple generation
Mob heads for players now show both skin layers
Buttons can now be placed on the ceiling
Lots and lots of other changes
LOTS AND LOTS of other changes
Removed Herobrine