Farmland can be placed on invalid locations

Same as MC-102986, but with farmland blocks.

You can pick block farmland and place it under blocks, when you then update the block (by placing a block next to it for example) the farmland will become dirt (intended).

Expected:
The farmland would instantly be replaced by dirt, not after a block update.

Steps to reproduce:

In creative pick block farmland.
Place a block (for example planks)
Place the farmland below it.
See it doesn't change to dirt.
Place a block besides the farmland.
See it change to dirt.

Fix: (Based off of Minecraft 1.11, MCP 9.35)

net.minecraft.block.BlockFarmland.java

public void onBlockAdded(World worldIn, BlockPos pos, IBlockState state)
    {
        super.onBlockAdded(worldIn, pos, state);
        this.func_190971_b(worldIn, pos);
    }

    private void func_190971_b(World worldIn, BlockPos pos)
    {
        if (worldIn.getBlockState(pos.up()).getMaterial().isSolid())
        {
        	worldIn.setBlockState(pos, Blocks.DIRT.getDefaultState());
        }
    }