Issue with v2v bin operation and indexes (SciRuby#501)

 - added a utility method to fall back to string-sort
   in case sort raise error
 - this metod called in v2v_binary method
 - impending test removed and passes

lib/daru/helpers/array.rb

@@ -36,5 +36,18 @@ def array_of?(array, match)
         !array.empty? &&
         array.all? { |el| match === el } # rubocop:disable Style/CaseEquality
     end
+
+    def sort_composite_data(array)
+      array.sort
+    rescue ArgumentError, TypeError => msg
+      case msg.to_s
+      when /comparison of Symbol with String failed/,
+        /comparison of Symbol with \d+ failed/,
+        /comparison of String with :.* failed/,
+        /comparison of Integer with :.* failed/,
+        /no implicit conversion from nil to integer/
+        return array.sort_by(&:to_s)
+      end
+    end
   end
 end

lib/daru/maths/arithmetic/vector.rb

@@ -92,7 +92,7 @@ def v2o_binary operation, other
 
         def v2v_binary operation, other, opts={}
           # FIXME: why the sorting?.. - zverok, 2016-05-18
-          index = (@index.to_a | other.index.to_a).sort
+          index = ArrayHelper.sort_composite_data(@index.to_a | other.index.to_a)
 
           elements = index.map do |idx|
             this = self.index.include?(idx) ? self[idx] : nil

spec/maths/arithmetic/vector_spec.rb

@@ -24,7 +24,7 @@
     end
 
     it "appropriately adds vectors with numeric and non-numeric indexes" do
-      pending "Need an alternate index implementation?"
+      # pending "Need an alternate index implementation?"
       v1 = Daru::Vector.new([1,2,3])
       v2 = Daru::Vector.new([1,2,3], index: [:a,:b,:c])