Fixed to write_headers option behavior when given no rows.

[Bug #9988][ruby-core:63375]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59640 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/csv.rb

@@ -1561,6 +1561,12 @@ class CSV
 
     # track our own lineno since IO gets confused about line-ends is CSV fields
     @lineno = 0
+
+    # make sure headers have been assigned
+    if header_row? and [Array, String].include? @use_headers.class and @write_headers
+      parse_headers  # won't read data for Array or String
+      self << @headers
+    end
   end
 
   #
@@ -1677,9 +1683,8 @@ class CSV
   #
   def <<(row)
     # make sure headers have been assigned
-    if header_row? and [Array, String].include? @use_headers.class
+    if header_row? and [Array, String].include? @use_headers.class and !@write_headers
       parse_headers  # won't read data for Array or String
-      self << @headers if @write_headers
     end
 
     # handle CSV::Row objects and Hashes

test/csv/test_interface.rb

@@ -295,6 +295,19 @@ class TestCSV::Interface < TestCSV
     end
   end
 
+  def test_write_headers_empty
+    File.unlink(@path)
+
+    CSV.open( @path, "wb", headers:       "b|a|c",
+                           write_headers: true,
+                           col_sep:       "|" ) do |csv|
+    end
+
+    File.open(@path, "rb") do |f|
+      assert_equal("b|a|c", f.gets.strip)
+    end
+  end
+
   def test_append  # aliased add_row() and puts()
     File.unlink(@path)