Change native.request to RawMessage

"The Native Object defines the native advertising opportunity available
for bid via this bid request. It will be included as a JSON-encoded
string in the bid request’s imp.native field or as a direct JSON object,
depending on the choice of the exchange. While OpenRTB 2.3/2.4 supports
only JSON-encoded strings, many exchanges have implemented a formal
object."

Author: Ev Dolzhenko

impression.go

@@ -17,6 +17,8 @@ var (
 // selling all ad positions on a given page as a bundle.  Each "imp" object has a required ID so that
 // bids can reference them individually.  An exchange can also conduct private auctions by
 // restricting involvement to specific subsets of seats within bidders.
+// The presence of Banner, Video, and/or Native objects
+// subordinate to the Imp object indicates the type of impression being offered.
 type Impression struct {
 	ID                string          `json:"id"` // A unique identifier for this impression
 	Banner            *Banner         `json:"banner,omitempty"`

native.go

@@ -10,7 +10,7 @@ import "encoding/json"
 // banner and/or video by also including as Imp subordinates the Banner and/or Video objects,
 // respectively. However, any given bid for the impression must conform to one of the offered types.
 type Native struct {
-	Request string          `json:"request"`         // Request payload complying with the Native Ad Specification.
+	Request json.RawMessage `json:"request"`         // Request payload complying with the Native Ad Specification.
 	Ver     string          `json:"ver,omitempty"`   // Version of the Native Ad Specification to which request complies; highly recommended for efficient parsing.
 	API     []int           `json:"api,omitempty"`   // List of supported API frameworks for this impression.
 	BAttr   []int           `json:"battr,omitempty"` // Blocked creative attributes