log expands negative numbers (just like Numbers)

The prinipal branch value is returned when the argument is negative:

log(-3) -> log(3) + I*pi  (previous behavior)
log(-sqrt(3)) -> log(sqrt(3)) + I*pi

Symbolic expressions which are negative are not rewritten.
[skip ci] fix test
Merge pull request #7750 from smichr/log-neg-arg

log expands negative numbers (just like Numbers)