reset spatial filter when no bounding box given

a spatial filter, once set, is never removed. 

import fiona
 
with fiona.open("test_uk.shp", 'r') as c:
    c1 = 0
    for f in c:
        c1 += 1
    c2 = 0
    for f in c.filter(bbox=(-1000, -1000, 0, 0)):
        c2 += 1
    c3 = 0
    for f in c:
        c3 += 1
    print(c1)
    print(c2)
    print(c3)

Output:
48
0
0