add support for connections to `write_xml`

[lbartnik]

write_xml accepts a file path as its second parameter. It could also accept a connection or even a broader list of outputs similar to cat()

[jimhester]

Sure we would probably accept a pull request implementing this, but I don't think we are going to implement it ourselves. as.character() will get you the text representation in memory if that is what you are after.

[lbartnik]

Interesting... I suppose it won't do formatting like write_xml(..., format = TRUE) would? I added this bc Hadley asked me. There's a JUnit reporter in testthat that needs to write to stdout() and currently I need to write to a tempfile() before passing to the final stream.

[hadley]

@jimhester we won't do it for this release, but it might be worthwhile to do next time, just to get more experience writing to connections (which ideally we should support anytime you can pass a filename)

[jimhester]

@lbartnik As of 0676f71 you can use as.character(..., format = TRUE) and get equivalent formatting to write_xml().