#4006 limit( (2*x+a)/(x+a), x, 0) doesn't check whether a=0

[Stavros Macrakis]

limit( (2*x+a)/(x+a), x, 0) always returns 1, without asking whether a=0 (in which case the correct result is 2).

Contributed by davewittemorris@sourceforge.

Tested in 5.46.0 / SBCL 2.2.5