Support block in CSV::Row#to_h (#356)

It will be consistent with Ruby's method, otherwise it has to be like
`row.entries.to_h { ... }`

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: tsymbalenkovlad

lib/csv/row.rb

@@ -637,6 +637,7 @@ def ==(other)
 
     # :call-seq:
     #   row.to_h -> hash
+    #   row.to_h {|key, value| ... } -> hash
     #
     # Returns the new \Hash formed by adding each header-value pair in +self+
     # as a key-value pair in the \Hash.
@@ -650,11 +651,34 @@ def ==(other)
     #   table = CSV.parse(source, headers: true)
     #   row = table[0]
     #   row.to_h # => {"Name"=>"Foo"}
+    #
+    # If a block is given, will call it with (key, value) arguments and use result as a hash entry:
+    #   source = "Name,Value\nfoo,1\nbar,2\nbaz,3\n"
+    #   table = CSV.parse(source, headers: true)
+    #   row = table[0]
+    #   row.to_h { |key, value| [key, "#{key}-#{value}"] } # => {"Name"=>"Name-foo", "Value"=>"Value-1"}
     def to_h
       hash = {}
-      each do |key, _value|
-        hash[key] = self[key] unless hash.key?(key)
+
+      if block_given?
+        each do |key, _value|
+          result = yield(key, self[key])
+          result_array = Array.try_convert(result)
+          raise TypeError, "wrong element type #{result.class} (expected array)" if result_array.nil?
+          raise ArgumentError, "wrong array length (expected 2, was #{result_array.size})" unless result_array.size == 2
+
+          key, value = result_array
+          next if hash.key?(key)
+
+          key.freeze if key.is_a?(String) && !key.frozen?
+          hash[key] = value
+        end
+      else
+        each do |key, _value|
+          hash[key] = self[key] unless hash.key?(key)
+        end
       end
+
       hash
     end
     alias_method :to_hash, :to_h

test/csv/test_row.rb

@@ -341,6 +341,29 @@ def test_to_hash
     end
   end
 
+  def test_to_hash_with_block_transform_values
+    hash = @row.to_hash { |k, v| [k, v**2] }
+    assert_equal({"A" => 1, "B" => 4, "C" => 9}, hash)
+    hash.each_key do |string_key|
+      assert_predicate(string_key, :frozen?)
+    end
+    assert_raise TypeError do
+      @row.to_hash { "foo" }
+    end
+    assert_raise ArgumentError do
+      @row.to_hash { [1] }
+    end
+  end
+
+  def test_to_hash_with_block_transform_entries
+    new_keys_map = {"A" => "A", "B" => "B", "C" => "B"}
+    hash = @row.to_hash { |k, v| [new_keys_map[k], v**2] }
+    assert_equal({"A" => 1, "B" => 4}, hash)
+    hash.each_key do |string_key|
+      assert_predicate(string_key, :frozen?)
+    end
+  end
+
   def test_to_csv
     # normal conversion
     assert_equal("1,2,3,4,\n", @row.to_csv)