When the coefficients are 0, the resulting cost for l1 and l2 weight decay
could return a float64 instead of a float32, when floatX=float32.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Add test on weight_decay with a coefficient of 0

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Complete test_weight_decay_0 test

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Merge pull request #973 from axeldavy/mlp_l1_l2_coeff0_fix

When the coefficients are 0, the resulting cost for l1 and l2 weight dec...