Fix bbox_to_mask to return np.float32

[knorth55]

np.testing.assert_equal does not check dtype!
I did not know that 😇 .

In [1]: import numpy as np

In [2]: a = np.array([1, 2, 3], dtype=np.float32)

In [3]: b = a.astype(np.float64)

In [4]: np.testing.assert_array_equal(a, b)

In [5]: a[2] = 4

In [6]: np.testing.assert_array_equal(a, b)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-6-e23e3200d0b5> in <module>()
----> 1 np.testing.assert_array_equal(a, b)

/usr/lib/python2.7/dist-packages/numpy/testing/utils.pyc in assert_array_equal(x, y, err_msg, verbose)
    805     """
    806     assert_array_compare(operator.__eq__, x, y, err_msg=err_msg,
--> 807                          verbose=verbose, header='Arrays are not equal')
    808 
    809 def assert_array_almost_equal(x, y, decimal=6, err_msg='', verbose=True):

/usr/lib/python2.7/dist-packages/numpy/testing/utils.pyc in assert_array_compare(comparison, x, y, err_msg, verbose, header, precision)
    731                                 names=('x', 'y'), precision=precision)
    732             if not cond:
--> 733                 raise AssertionError(msg)
    734     except ValueError:
    735         import traceback

AssertionError: 
Arrays are not equal

(mismatch 33.3333333333%)
 x: array([ 1.,  2.,  4.], dtype=float32)
 y: array([ 1.,  2.,  3.])

[Hakuyume]

LGTM