fix series expansion of abs

fix series expansion of abs

This improves series of abs around zero. Error term
is returned only if needed
before:
In [23]: abs(x).nseries(x,0,2)
Out[23]: x + O(x**2)
now:
In [27]: abs(x).nseries(x, 0, 2)
Out[27]: x
Expansion is now computed also for nonzero arguments of abs,
In [34]: abs(sin(x+1)+1).nseries(x,0,2)
Out[34]: 1 + x⋅cos(1) + sin(1) + O(x**2)
Now the abs(sin(x+1)) limit works
In [37]: abs(sin(x+1)).limit(x,0)
Out[37]: sin(1)

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>