mapper: correct text/binary typo

Fixes a panic when attempting to make use of pointers to types that
implement encoding.BinaryUnmarshaler.  e.g.:

    type SomeBinaryFlag struct{}

    func (f *SomeBinaryFlag) UnmarshalBinary(data []byte) error {
	    // ...
	    return nil
    }

    var cli struct {
	    Binary *SomeBinaryFlag
    }

mapper.go

@@ -84,7 +84,7 @@ func (m *binaryUnmarshalerAdapter) Decode(ctx *DecodeContext, target reflect.Val
 	if err != nil {
 		return err
 	}
-	if target.Type().Implements(textUnmarshalerType) {
+	if target.Type().Implements(binaryUnmarshalerType) {
 		return target.Interface().(encoding.BinaryUnmarshaler).UnmarshalBinary([]byte(value))
 	}
 	return target.Addr().Interface().(encoding.BinaryUnmarshaler).UnmarshalBinary([]byte(value))