Forbids use of XML::Writer.new.
XML::Writer.(io|string|document|string) class methods should be used instead
of a direct instanciation.
Revise flushing:
* XML::Writer.flush: for a in memory writer, it returns the current content
(as String) of internal buffer. In others cases, it returns the number of
written bytes. Besides, if its unique argument is true (default), the internal
buffer is erased for a in memory writer.
* XML::Writer.result: flush output before fetching result
Change default value for pe argument from Qnil to Qundef in "wrapper": a
nil value, passed by a user, could result in undefined behavior (may
segfault)
Merge pull request #54 from julp/writer_class_fix

XML::Writer class multiple fixes