remove a few that generate lots of warnings on ruby.h:
- -Wcast-qual
- -Wconversion

add -Wextra back (it was removed in 2008)

to fix compiler warnings.

the node->type check is polymorphic, and so a macro makes more sense to
me (so we get some compile-time checking) than force-typecasting.

xmlNewCDataBlock() takes ptr and len, so we can use RSTRING_LEN()
instead of strlen(). strlen() was introduced in commit c766f6e
to fix a segfault, but it's enough to execute StringValuePtr() and
RSTRING_LEN() in order to avoid invalid memory access.

…arnings

Flavorjones fix compilation warnings