encoding: Restore old lookup order in xmlOpenCharEncodingHandler

When looking up encodings with xmlLookupCharEncodingHandler, the
returned handler can have a different name than requested
(capitalization, internal aliases). This should eventually be fixed.
For now we revert part of commit 5b893fa9, start the lookup with
xmlFindHandler and add an explicit check for UTF-8.

Should fix the encoding name issue mentioned in #749.

testparser.c

@@ -50,7 +50,8 @@ testUnsupportedEncoding(void) {
     xmlFreeDoc(doc);
 
     error = xmlGetLastError();
-    if (error->code != XML_ERR_UNSUPPORTED_ENCODING ||
+    if (error == NULL ||
+        error->code != XML_ERR_UNSUPPORTED_ENCODING ||
         error->level != XML_ERR_WARNING ||
         strcmp(error->message, "Unsupported encoding: #unsupported\n") != 0)
     {