Correct the calculation of relative paths in the html reports

Add spec for the conversion to relative paths in the html formatter.

lib/cucumber/formatter/html.rb

@@ -44,9 +44,10 @@ def embed(src, mime_type, label)
       def embed_image(src, label)
         id = "img_#{@img_id}"
         @img_id += 1
-        if @io.respond_to?(:absolute_path) and File.exist?(src)
-          src = Pathname.new(File.absolute_path(src)).relative_path_from(@io.absolute_path)
-        end
+        if @io.respond_to?(:path) and File.file?(src)
+          out_dir = Pathname.new(File.dirname(File.absolute_path(@io.path)))
+          src = Pathname.new(File.absolute_path(src)).relative_path_from(out_dir)
+        end        
         @builder.span(:class => 'embed') do |pre|
           pre << %{<a href="" onclick="img=document.getElementById('#{id}'); img.style.display = (img.style.display == 'none' ? 'block' : 'none');return false">#{label}</a><br>&nbsp;
           <img id="#{id}" style="display: none" src="#{src}"/>}

spec/cucumber/formatter/html_spec.rb

@@ -28,6 +28,35 @@ module Formatter
         }).not_to raise_error
       end
 
+      describe "when writing the report to a file" do
+        before(:each) do
+          allow(@out).to receive(:respond_to?).with(:path, false).and_return(true)
+          expect(@out).to receive(:respond_to?).with(:path).and_return(true)
+          expect(@out).to receive(:path).and_return('out/file.html')
+          run_defined_feature
+          @doc = Nokogiri.HTML(@out.string)
+        end
+
+        describe "with a step that embeds a snapshot" do
+          define_steps do
+            Given(/snap/) { 
+              RSpec::Mocks.allow_message(File, :file?) { true }
+              embed('out/snapshot.jpeg', 'image/jpeg')
+            }
+          end
+
+          define_feature(<<-FEATURE)
+          Feature:
+            Scenario:
+              Given snap
+            FEATURE
+
+          it "converts the snapshot path to a relative path" do
+            expect(@doc.css('.embed img').first.attributes['src'].to_s).to eq "snapshot.jpeg"
+          end
+        end
+      end
+
       describe "given a single feature" do
         before(:each) do
           run_defined_feature