sjsxp issue 57: XMLEventReaderImpl.getElementText() and nextTag() fail to update last event

The getElementText() and nextTag() methods of XMLEventReaderImpl advance the
position in the stream but do not update fLastEvent if there was no peeked event.

As such, a call of the form:

[1] peek()
[2] getElementText()
[3] nextTag()
[4] getElementText()

will fail with the exception "parser must be on START_ELEMENT to read next text"
because the fLastEvent was left pointing to an END_ELEMENT in [2] above and not
updated in [3].