func (e *UnexpectedResponseError) String() string {
	return fmt.Sprintf("UnexpectedResponseError URL=%s ExpectedOneOf=%#v Got=%d Error: %s", e.URL, e.Expected, e.Actual, string(e.Data))
}
String() converts the error into a human-readable, logfmt-compliant string.
See http://godoc.org/github.com/kr/logfmt for details on logfmt formatting.
package mailgun
import (
	"fmt"
)
const MailgunGoUserAgent = "mailgun-go/1.0.0"
type UnexpectedResponseError struct {
	Expected []int
	Actual   int
	URL      string
	Data     []byte
}