Fix xml_parse_result::encoding for empty document load

The new early-out for size==0 has a subtly different behavior from the
main path: encoding is left at auto instead of being set to the inferred
encoding (which is always UTF-8 for empty inputs).

In general, xml_parse_result::encoding is a bit inconsistent for error
returns: the parse() path sets it including errors, but other paths
don't. That said, this is somewhat defensible since there's some difference
between not having parsed anything and having parsed something.

In any case, before the early out, fragment parse of an empty document
would return UTF-8 encoding, so this change restores the behavior and
adds a test for it.

Author: zeux

src/pugixml.cpp

@@ -4850,7 +4850,12 @@ PUGI_IMPL_NS_BEGIN
 		auto_deleter<void> contents_guard(own ? contents : NULL, xml_memory::deallocate);
 
 		// early-out for empty documents to avoid buffer allocation overhead
-		if (size == 0) return make_parse_result((options & parse_fragment) ? status_ok : status_no_document_element);
+		if (size == 0)
+		{
+			xml_parse_result result = make_parse_result((options & parse_fragment) ? status_ok : status_no_document_element);
+			result.encoding = buffer_encoding;
+			return result;
+		}
 
 		// get private buffer
 		char_t* buffer = NULL;

tests/test_document.cpp

@@ -1318,6 +1318,15 @@ TEST(document_load_buffer_empty_fragment)
 	}
 }
 
+TEST(document_load_buffer_empty_encoding)
+{
+	xml_document doc;
+	xml_parse_result result = doc.load_buffer(0, 0, parse_fragment);
+
+	CHECK(result.status == status_ok);
+	CHECK(result.encoding == encoding_utf8);
+}
+
 TEST(document_load_buffer_null)
 {
 	xml_document doc;