BULAHAWA, Somalia — Muslim extremists seeking evidence that a Somali man had converted from Islam to Christianity shot him dead Aug. 18 near the Somali border with Kenya, according to underground Christians in the war-torn nation. Al Shabaab rebels killed 41-year-old Ahmed Matan in Bulahawa, Somalia, according to Abdikadir Abdi Ismael, a former leader of a secret Christian fellowship in…