Update WSGIWrapper to be spec compliant (pep 333/3333).

The commit contains the following changes:

 * Error out early if the application forgets to call start_response
 * Send a proper content-length if the iterable returned has only one
   item
 * Always call `close()` on the returned iterable if it exists.

src/hypercorn/app_wrappers.py

@@ -78,31 +78,63 @@ async def handle_http(
             environ = _build_environ(scope, body)
         except InvalidPathError:
             await send({"type": "http.response.start", "status": 404, "headers": []})
+            await send({"type": "http.response.body", "body": b"", "more_body": False})
         else:
             await sync_spawn(self.run_app, environ, partial(call_soon, send))
-        await send({"type": "http.response.body", "body": b"", "more_body": False})
 
     def run_app(self, environ: dict, send: Callable) -> None:
         headers: List[Tuple[bytes, bytes]]
+        headers_set: bool = False
+        headers_sent: bool = False
+        implicit_content_length: bool = False
         status_code: Optional[int] = None
 
         def start_response(
             status: str,
             response_headers: List[Tuple[str, str]],
             exc_info: Optional[Exception] = None,
         ) -> None:
-            nonlocal headers, status_code
+            nonlocal headers, headers_set, status_code
 
             raw, _ = status.split(" ", 1)
             status_code = int(raw)
             headers = [
                 (name.lower().encode("ascii"), value.encode("ascii"))
                 for name, value in response_headers
             ]
+            headers_set = True
+
+        def send_headers(content_length: int):
+            nonlocal headers, headers_sent
+            if not headers_set:
+                raise AssertionError("missing call to start_response")
+            if implicit_content_length:  # We can determine the content-length ourself if not set
+                for name, _ in headers:
+                    if name == b"content-length":
+                        break
+                else:  # No content-length set by the application
+                    headers.append((b"content-length", str(content_length).encode("ascii")))
             send({"type": "http.response.start", "status": status_code, "headers": headers})
+            headers_sent = True
 
-        for output in self.app(environ, start_response):
-            send({"type": "http.response.body", "body": output, "more_body": True})
+        def send_body(data: bytes):
+            if not headers_sent:
+                send_headers(len(data))
+            send({"type": "http.response.body", "body": data, "more_body": True if data else False})
+
+        response_body_iter = self.app(environ, start_response)
+
+        if hasattr(response_body_iter, "__len__") and len(response_body_iter) == 1:
+            implicit_content_length = True
+
+        try:
+            for output in response_body_iter:
+                if output:
+                    send_body(output)
+            send_body(b"")
+        finally:
+            if hasattr(response_body_iter, "close"):
+                response_body_iter.close()
 
 
 def _build_environ(scope: HTTPScope, body: bytes) -> dict: