fix reference leak in the capi export function, add testcase

Cython/Compiler/ModuleNode.py

@@ -2128,6 +2128,7 @@ static int __Pyx_ExportFunction(const char *name, void *f, const char *sig) {
         goto bad;
     if (PyDict_SetItemString(d, name, p) < 0)
         goto bad;
+    Py_DECREF(p);
     Py_DECREF(d);
     return 0;
 bad:

tests/run/capiimpl.pyx

+__doc__ = u"""
+>>> import sys
+>>> sys.getrefcount(Foo.__pyx_vtable__)
+2
+>>> sys.getrefcount(__pyx_capi__['spam'])
+2
+"""
+
+cdef public api class Foo [type FooType, object FooObject]:
+    cdef void bar(self):
+        pass
+
+cdef api void spam():
+    pass