Replace if statements with switch when using enums in ixml.
Remove uneeded initialization in ixmlAttr_init, Parser_init and
ixmlNode_init which was added by wrongly added in commit
06660b6383c438e4e2c9ca9854077cecc4da9e5d.
(cherry picked from commit d48d73720bd325062c4d3b9ce85f3944be4f562d)