Adding additionalHeaders property to OneSignalRequest

This property contains key value pairs for setting headers on a the urlRequest.

Author: emawby

iOS_SDK/OneSignalSDK/Source/OneSignalRequest.h

@@ -38,6 +38,7 @@
 @property (nonatomic) HTTPMethod method;
 @property (strong, nonatomic, nonnull) NSString *path;
 @property (strong, nonatomic, nullable) NSDictionary *parameters;
+@property (strong, nonatomic, nullable) NSDictionary<NSString *, NSString *> *additionalHeaders;
 @property (nonatomic) int reattemptCount;
 @property (nonatomic) BOOL dataRequest; //false for JSON based requests
 -(BOOL)missingAppId; //for requests that don't require an appId parameter, the subclass should override this method and return false

iOS_SDK/OneSignalSDK/Source/OneSignalRequest.m

@@ -60,6 +60,10 @@ -(NSMutableURLRequest *)urlRequest {
 
     let request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]];
 
+    for (NSString *key in self.additionalHeaders) {
+        [request setValue:self.additionalHeaders[key] forHTTPHeaderField:key];
+    }
+    
     if (!self.dataRequest)
         [request setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];