fdo#74595 Make HTML detection to follow specs

See http://www.w3.org/TR/html-markup/documents.html#conformant-html

Change-Id: Id2835f20baad20bcb874df5e6f35a189c320367b
Reviewed-on: https://gerrit.libreoffice.org/8079Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

filter/source/htmlfilterdetect/filterdetect.cxx

@@ -53,7 +53,7 @@ bool isHTMLStream(const OString& aStreamHeader)
     for ( i = 0; i < nLength; ++i, ++pHeader )
     {
         char c = *pHeader;
-        if ( c == ' ' || c == '\n' || c == '\t' )
+        if ( c == ' ' || c == '\n' || c == '\t' || c == '\r' || c == '\f' )
         {
             if ( dp == TagOpened )
                 return false; // Invalid: Should start with a tag name
@@ -76,8 +76,8 @@ bool isHTMLStream(const OString& aStreamHeader)
         }
         else if ( c == '!' )
         {
-            if ( i == 1 && dp == TagOpened )
-                return true; // "<!" at the very beginning of the file
+            if ( dp == TagOpened )
+                return true; // "<!" - DOCTYPE or comments block
             else
                 return false; // Invalid: '!' before '<' or inside tag name
         }