[Bug-gsl] gsl_sf_hyperg_1F1 bug

[Andrew Silberfarb]

This function erroneously negates its return values for some specific 
arguments.

Specifically when
a is a negative integer
b = -.5

The return value is incorrect.  Presumably this is due to special case 
handling in the function as for negative integral a the 1F1 
hypergeometric function reduces to a Laguerre polynomial.

This error can also be seen by plotting the function value versus a, 
and noticing a discontinuity at negative integer values.