reset spatial filter when no bounding box given

a spatial filter, is once set, never removed. 
import fiona

if __name__ == '__main__':
    with fiona.open("test_uk.shp", 'r') as c:
        c1 = 0
        for f in c:
#             print(f)
            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