Fix crash #174 by duplicating curl handle

Wrap libcurl to bypass lack of covariance support

curl.go

@@ -10,4 +10,16 @@ type Curler interface {
 	Setopt(opt int, param interface{}) error
 	Perform() error
 	Getinfo(info curl.CurlInfo) (interface{}, error)
+	Duphandle() Curler
+}
+
+// Curl is a libcurl wrapper used to make the Curler interface work even though
+// golang currently does not support covariance (see https://github.com/golang/go/issues/7512)
+type Curl struct {
+	*curl.CURL
+}
+
+// Duphandle wraps curl.Duphandle
+func (c *Curl) Duphandle() Curler {
+	return &Curl{c.CURL.Duphandle()}
 }