ENUNCIATE-748 objc parser: fix bug self-close xml without wrapper

The Objective-C parser is not able to handle XML properly when parsing a XML with <b>self-closing tag without a wrapper.</b>

Example without wrapper (bug):
	<bus>
		<seatRows>
			<seats>
				<number>1</number>
			</seats>
			<seats/>
			<seats>
				<number>3</number>
			</seats>
		</seatRows>
	</bus>

Example with wrapper (working):
	<bus>
		<seatRows>
			<listSeat>
				<seats>
					<number>1</number>
				</seats>
				<seats/>
				<seats>
					<number>3</number>
				</seats>
			</listSeat>
		</seatRows>
	</bus>

Without wapping, the generated parser will call 2 times `xmlTextReaderAdvanceToNextStartOrEndElement` when it find a self-closing tag.<br />
As a result it jump the self-close tag and an extra element.

Following [TristanBrismontier](https://github.com/stoicflame/enunciate/pull/17) work, I implemented a unit test who reproduce the bug and correct it.