Handle well-indented XML including comments

The root cause for #782 was misuse of xmerl_scan:string/2, which doesn't emit spaces, comments between actual xml elements. It leaves them as #xmlText{} and #xmlComment{} - This commit changes to ignore them at walking around parsed dom tree. This diffset also includes a small refactoring that wraps all xmerl_scan:string/2 to one riak_cs_xml funcsion to control its usage.

This also adds regression test as simple eunit test case.

backport of #793