Marshal produces output that Unmarshal cannot decode

[rogpeppe]

After commit 72c33f6, this code fails.
Even if the yaml package was changed to quote "<" on output, we'd
still have a problem, as this character has not been quoted historically,
so it can fail to parse old files (or output produced by other yaml producers,
such as python's yaml module).

package main

import (
    "log"
    "fmt"
    "gopkg.in/yaml.v1"
)

func main() {
    d := struct {
        Net string
    }{
        `<foo>=bar`,
    }
    data, err := yaml.Marshal(d)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%s", data)
    var x interface{}
    err = yaml.Unmarshal(data, &x)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%#v", x)
}

[rogpeppe]

Fixed by feb4ca7