Fix internal link handling for dirs w/o root_dir

lib/html-proofer/element.rb

@@ -181,7 +181,11 @@ def file_path
       path_dot_ext = path + @check.options[:extension] if @check.options[:assume_extension]
 
       base = if absolute_path?(path) # path relative to root
-               @check.options[:root_dir] || File.dirname(@check.src)
+               if File.directory?(@check.src)
+                 @check.options[:root_dir] || @check.src
+               else
+                 @check.options[:root_dir] || File.dirname(@check.src)
+               end
              elsif File.exist?(File.expand_path(path, @check.src)) || File.exist?(File.expand_path(path_dot_ext, @check.src)) # relative links, path is a file
                File.dirname(@check.path)
              elsif File.exist?(File.join(File.dirname(@check.path), path)) || File.exist?(File.join(File.dirname(@check.path), path_dot_ext)) # rubocop:disable Lint/DuplicateBranch; relative links in nested dir, path is a file
@@ -190,6 +194,7 @@ def file_path
                @check.path
              end
       file = File.join(base, path)
+
       if @check.options[:assume_extension] && File.file?("#{file}#{@check.options[:extension]}")
         file = "#{file}#{@check.options[:extension]}"
       elsif File.directory?(file) && !unslashed_directory?(file) # implicit index support

spec/html-proofer/fixtures/links/internals/direct_link.html

@@ -0,0 +1,2 @@
+<a href="page.html"></a>
+<a href="/page.html"></a>

spec/html-proofer/links_spec.rb

@@ -662,4 +662,10 @@
     proofer = run_proofer(file, :file)
     expect(proofer.failed_tests.first).to match(/failed/)
   end
+
+  it 'works for a direct link through directory' do
+    file = "#{FIXTURES_DIR}/links/internals"
+    proofer = run_proofer(file, :directory)
+    expect(proofer.failed_tests).to eq []
+  end
 end