inconsistent http_listener::close

[garethsb]

The different constructors for web::http::experimental::listener::details::http_listener_impl initialize the m_close_task differently.

cpprestsdk/Release/include/cpprest/http_listener.h

Lines 203 to 205 in 07cf589

	// Used to record that the listener is closed.
	bool m_closed;
	pplx::task<void> m_close_task;

The default constructor that is visible to SDK users since it is inline in the header initializes it like so:

cpprestsdk/Release/include/cpprest/http_listener.h

Line 170 in 07cf589

http_listener_impl() : m_closed(true), m_close_task(pplx::task_from_result()) {}

However, the other constructors leave the task default initialized:

cpprestsdk/Release/src/http/listener/http_listener.cpp

Lines 57 to 66 in 07cf589

	details::http_listener_impl::http_listener_impl(http::uri address) : m_uri(std::move(address)), m_closed(true)
	{
	check_listener_uri(m_uri);
	}
	details::http_listener_impl::http_listener_impl(http::uri address, http_listener_config config)
	: m_uri(std::move(address)), m_config(std::move(config)), m_closed(true)
	{
	check_listener_uri(m_uri);
	}

A default constructed task behaves quite differently, so this seems like an oversight.