Time out requests the server never answers

HTTPRequest has no timeout by default, so a server that accepts the
connection but never responds left song downloads waiting forever.
Requests now fail after 120 seconds, which triggers the normal retry.

http_transport.gd

@@ -4,6 +4,10 @@ extends RefCounted
 ##
 ## Tests subclass this to answer requests without a network connection.
 
+## Seconds before a request is given up and reported as failed. This covers the whole
+## download, so it is generous enough for long songs on a slow connection.
+var timeout_seconds: float = 120.0
+
 var _owner: Node
 
 
@@ -12,10 +16,11 @@ func _init(owner: Node) -> void:
 
 
 ## Requests [param url] and calls [param on_complete] with
-## [code](response_code: int, body: PackedByteArray)[/code]. Connection errors are
-## reported with response code 0.
+## [code](response_code: int, body: PackedByteArray)[/code]. Connection errors and
+## timeouts are reported with response code 0.
 func request(url: String, method: int, on_complete: Callable) -> void:
     var http_request: HTTPRequest = HTTPRequest.new()
+    http_request.timeout = timeout_seconds
     _owner.add_child(http_request)
     http_request.request_completed.connect(
         func(