Use headers property. (#3264)

Source/ParameterEncoding.swift

@@ -168,8 +168,8 @@ public struct URLEncoding: ParameterEncoding {
                 urlRequest.url = urlComponents.url
             }
         } else {
-            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
-                urlRequest.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
+            if urlRequest.headers["Content-Type"] == nil {
+                urlRequest.headers.update(.contentType("application/x-www-form-urlencoded; charset=utf-8"))
             }
 
             urlRequest.httpBody = Data(query(parameters).utf8)
@@ -265,8 +265,8 @@ public struct JSONEncoding: ParameterEncoding {
         do {
             let data = try JSONSerialization.data(withJSONObject: parameters, options: options)
 
-            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
-                urlRequest.setValue("application/json", forHTTPHeaderField: "Content-Type")
+            if urlRequest.headers["Content-Type"] == nil {
+                urlRequest.headers.update(.contentType("application/json"))
             }
 
             urlRequest.httpBody = data
@@ -293,8 +293,8 @@ public struct JSONEncoding: ParameterEncoding {
         do {
             let data = try JSONSerialization.data(withJSONObject: jsonObject, options: options)
 
-            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
-                urlRequest.setValue("application/json", forHTTPHeaderField: "Content-Type")
+            if urlRequest.headers["Content-Type"] == nil {
+                urlRequest.headers.update(.contentType("application/json"))
             }
 
             urlRequest.httpBody = data