resolves #1916 for real; add test

- actually make sourcemap property writable
- add test to verify sourcemap property is writable

lib/asciidoctor/document.rb

@@ -116,7 +116,7 @@ def to_s
   attr_reader :compat_mode
 
   # Public: Get or set the Boolean flag that indicates whether source map information should be tracked by the parser
-  attr :sourcemap
+  attr_accessor :sourcemap
 
   # Public: Get the Hash of document references
   attr_reader :references

test/document_test.rb

@@ -343,6 +343,18 @@ def [](key)
       assert_equal 1, section_1.lineno
     end
 
+    test 'should allow sourcemap option on document to be modified' do
+      doc = Asciidoctor.load_file fixture_path('sample.asciidoc'), :parse => false
+      doc.sourcemap = true
+      doc = doc.parse
+
+      section_1 = doc.sections[0]
+      assert_equal 'Section A', section_1.title
+      refute_nil section_1.source_location
+      assert_equal 'sample.asciidoc', section_1.file
+      assert_equal 10, section_1.lineno
+    end
+
     test 'find_by should return Array of blocks anywhere in document tree that match criteria' do
       input = <<-EOS
 = Document Title