Offset is empty with usetex when offset is equal to 1

Example:

    from pylab import *
    rc('text',usetex=True)
    t=arange(1,1+1e-6,1e-7)
    f = lambda x: 1e6*(x-1)
    plot(t,vectorize(f)(t))
    show()

![Missing offset](http://i1144.photobucket.com/albums/o488/idrathermeetyouatthebottom/bug_zps6041be28.png)

The problem arises in ScalarFormatter._formatSciNotation; a solution could be replacing

    if significand == '1':

with 

    if significand == '1' and not exponent == '':