wininet/tests: Add a test to show that the Content-Length header is not updated when a request is reused.

This shouldn't be needed as the device is opened in non-blocking mode,
and it appears this function is unimplemented on PCBSD.

On Yosemite, in full-screen mode, Cocoa adds child windows of its own to our
windows.  These windows are, of course, not instances of WineWindow.  So, when
we call WineWindow-specific methods on them, it throws exceptions.

Based on a patch by Huw Campbell.