force :xhtml if options[:mime_type] == 'text/xml'

If options[:mime_type] is defined and the value == 'text/xml'
then an instanced Haml::Engine will always use :xhtml

lib/haml/engine.rb

@@ -101,6 +101,8 @@ module Haml
       @options.merge! options.reject {|k, v| v.nil?}
       @index = 0
 
+      @options[:format] = :xhtml if @options[:mime_type] == 'text/xml'
+
       unless [:xhtml, :html4, :html5].include?(@options[:format])
         raise Haml::Error, "Invalid output format #{@options[:format].inspect}"
       end

test/engine_test.rb

@@ -1528,6 +1528,36 @@ HAML
       render("%div{data_val, :data => {:foo => 'blip', :brat => 'wurst'}}"))
   end
 
+  def test_xml_doc_using_html5_format_and_mime_type
+    assert_equal(<<XML, render(<<HAML, { :format => :html5, :mime_type => 'text/xml' }))
+<?xml version='1.0' encoding='utf-8' ?>
+<root>
+  <element />
+  <hr />
+</root>
+XML
+!!! XML
+%root
+  %element/
+  %hr
+HAML
+  end
+
+  def test_xml_doc_using_html4_format_and_mime_type
+    assert_equal(<<XML, render(<<HAML, { :format => :html4, :mime_type => 'text/xml' }))
+<?xml version='1.0' encoding='utf-8' ?>
+<root>
+  <element />
+  <hr />
+</root>
+XML
+!!! XML
+%root
+  %element/
+  %hr
+HAML
+  end
+
   # New attributes
 
   def test_basic_new_attributes