Fix complex addition handling in C extension (#50)

Fix the sum path for complex initial values, which was implemented but
not functioning correctly.

Author: kojix2

ext/enumerable/statistics/extension/statistics.c

@@ -163,7 +163,7 @@ complex_add(VALUE self, VALUE other)
 
     real = f_add(RCOMPLEX(self)->real, other);
 
-    return complex_new(CLASS_OF(self), real, RCOMPLEX(other)->imag);
+    return complex_new(CLASS_OF(self), real, RCOMPLEX(self)->imag);
   }
   return rb_num_coerce_bin(self, other, idPLUS);
 }

spec/array_spec.rb

@@ -3,6 +3,12 @@
 require 'delegate'
 
 RSpec.describe Array do
+  describe '#sum' do
+    it 'sums with a complex init' do
+      expect([1, 2, 3].sum(Complex(1, 2))).to eq(Complex(7, 2))
+    end
+  end
+
   describe '#mean' do
     let(:ary) { [] }
     let(:block) { nil }