Compute single-value-type correctly in the absence of required values

 * For the longest time (at least 2003), we didn't take defaulting
   into account and did not union the single-value type with NULL.
   For some reason, the issue didn't manigest itself until we improved
   code generation for EQ/EQL this month.

   Thanks to Attila Lendvai for the reasonable test case.

 * Also, fix a typo in a VOP name for EQ of fixnum values.