`read_xml(x)` throws an error if `x` is a `textConnection()`

[trevorld]

Currently I observe that read_xml(x) throws an error if x is a textConnection().

library("xml2")
s <- '<?xml version="1.0" encoding="UTF-8"?>\n<outer>\n<inner>Inner</inner>\n</outer>'
con <- textConnection(s)
x <- read_xml(con)

Error in readBin(3L, "raw", 65536L) : 
  can only read from a binary connection

I'm aware you could use x <- read_xml(paste(readLines(con), sep = "\n")) as a workaround but this seems like an easy case to support.

I'm putting together a pull request that adds a read_xml.textConnection() S3 method.