feat: allow 'auto' value for the 'dir' attribute of Package Documents

Fix #1220

src/main/resources/com/adobe/epubcheck/schema/30/package-30.rnc

@@ -130,4 +130,4 @@
    opf.id.attr = attribute id { datatype.ID }
    opf.i18n.attrs = opf.xml.lang.attr? & opf.dir.attr?
    opf.xml.lang.attr = attribute xml:lang { "" | datatype.string.nonempty }
-   opf.dir.attr = attribute dir { 'ltr' | 'rtl' }
+   opf.dir.attr = attribute dir { 'ltr' | 'rtl' | 'auto' }

src/test/resources/epub3/files/package-document/attr-dir-auto-valid.opf

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<package dir="auto" xmlns="http://www.idpf.org/2007/opf" version="3.0" unique-identifier="uid"
+    xmlns:dc="http://purl.org/dc/elements/1.1/">
+    <metadata>
+        <dc:title>Title</dc:title>
+        <dc:language>en</dc:language>
+        <dc:identifier id="uid">NOID</dc:identifier>
+        <meta property="dcterms:modified">2019-01-01T12:00:00Z</meta>
+    </metadata>
+    <manifest>
+        <item id="t001" href="contents.xhtml" properties="nav" media-type="application/xhtml+xml"/>
+    </manifest>
+    <spine>
+        <itemref idref="t001"/>
+    </spine>
+</package>

src/test/resources/epub3/package-document.feature

@@ -87,6 +87,10 @@ Feature: EPUB 3 ▸ Packages ▸ Package Document Checks
     Then error RSC-005 is reported 2 times (once for each ID)
     And no other errors or warnings are reported
 
+  Scenario: the 'dir' attribute value can be 'auto' 
+    When checking file 'attr-dir-auto-valid.opf'
+    Then no errors or warnings are reported
+
   Scenario: the 'xml:lang' attribute can be empty
     When checking file 'attr-lang-empty-valid.opf'
     Then no other errors or warnings are reported