Fix #add_row for DF with multi index (#418)

.rubocop.yml

@@ -12,6 +12,7 @@ AllCops:
     - 'vendor/**/*'
     - 'benchmarks/*'
     - 'profile/*'
+    - 'tmp/*'
   DisplayCopNames: true
   TargetRubyVersion: 2.0
 

lib/daru/dataframe.rb

@@ -533,7 +533,7 @@ def []=(*args)
     end
 
     def add_row row, index=nil
-      self.row[index || @size] = row
+      self.row[*(index || @size)] = row
     end
 
     def add_vector n, vector

spec/dataframe_spec.rb

@@ -1587,6 +1587,22 @@
       }
     end
 
+    context 'with mulitiindex DF' do
+      subject(:data_frame) {
+        Daru::DataFrame.new({b: [11,12,13], a: [1,2,3],
+          c: [11,22,33]}, order: [:a, :b, :c],
+          index: Daru::MultiIndex.from_tuples([[:one, :two], [:one, :three], [:two, :four]]))
+      }
+
+      before { data_frame.add_row [100,200,300], [:two, :five] }
+
+      it { is_expected.to eq(Daru::DataFrame.new({
+          b: [11,12,13,200], a: [1,2,3,100],
+          c: [11,22,33,300]}, order: [:a, :b, :c],
+          index: Daru::MultiIndex.from_tuples([[:one, :two], [:one, :three], [:two, :four], [:two, :five]])))
+      }
+    end
+
     it "allows adding rows after making empty DF by specfying only order" do
       df = Daru::DataFrame.new({}, order: [:a, :b, :c])
       df.add_row [1,2,3]