Fix restoring default tensor type after call .image(...)

[aropan]

Non-working example:

torch.setdefaulttensortype('torch.FloatTensor')
print(torch.getdefaulttensortype())
display = require('display')
display.image({torch.Tensor(1, 1, 1)})
print(torch.getdefaulttensortype())

Output:

torch.FloatTensor
torch.DoubleTensor

After fix and correct output:

torch.FloatTensor
torch.FloatTensor

[aropan]

@szym do merge as soon as can plz.