WebConnectionStream.Length now consistently throws a NotSupportedException.

The behaviour of the Length implementation was not consistent with the expected and documented behaviour of this property:
	CanSeek always returns false, so Length should always throw an exception.
	In all cases, it should *never* return -1, as it was the case when the Content-Length header could not be parsed.

See this bug on the Xamarin bug tracker for the discussion and an example of why the previous behaviour was a problem. https://bugzilla.xamarin.com/show_bug.cgi?id=15671