xml2_serialize() should create and open a connection if given a filename.

[jimhester]

Issue Description and Expected Result

Currently connection is passed directly to base::serialize, which expects an open connection.

Reproducible Example

library(xml2)
x <- read_xml("<x><y/><z/></x>")
xml_serialize(x, "test")
#> Error in serialize(structure(as.character(object, ...), class = "xml_serialized_document"), : 'connection' must be a connection