Add option `overwrite` for #173

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobConfig.java

     public String key;
     public String mime;
     public Boolean auto;
+    public Boolean overwrite = true;
     public long timeout = 60000;
     public Boolean increment = false;
     public ReadableArray binaryContentTypes = null;
         }
         if(options.hasKey("binaryContentTypes"))
             this.binaryContentTypes = options.getArray("binaryContentTypes");
+        if(this.path != null && path.toLowerCase().contains("?append=true")) {
+            this.overwrite = false;
+        }
+
+        if(options.hasKey("overwrite"))
+            this.overwrite = options.getBoolean("overwrite");
         this.key = options.hasKey("key") ? options.getString("key") : null;
         this.mime = options.hasKey("contentType") ? options.getString("contentType") : null;
         this.increment = options.hasKey("increment") ? options.getBoolean("increment") : false;

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

                                         RNFetchBlob.RCTContext,
                                         taskId,
                                         originalResponse.body(),
-                                        destPath);
+                                        destPath,
+                                        options.overwrite);
                                 break;
                             default:
                                 extended = new RNFetchBlobDefaultResp(

src/android/src/main/java/com/RNFetchBlob/Response/RNFetchBlobFileResp.java

     ReactApplicationContext rctContext;
     FileOutputStream ofStream;
 
-    public RNFetchBlobFileResp(ReactApplicationContext ctx, String taskId, ResponseBody body, String path) throws IOException {
+    public RNFetchBlobFileResp(ReactApplicationContext ctx, String taskId, ResponseBody body, String path, boolean overwrite) throws IOException {
         super();
         this.rctContext = ctx;
         this.mTaskId = taskId;
         assert path != null;
         this.mPath = path;
         if (path != null) {
-            boolean appendToExistingFile = path.contains("?append=true");
+            boolean appendToExistingFile = !overwrite;
             path = path.replace("?append=true", "");
             mPath = path;
             File f = new File(path);

src/ios/RNFetchBlobNetwork.m

             {
                 [fm createDirectoryAtPath:folder withIntermediateDirectories:YES attributes:NULL error:nil];
             }
+            BOOL overwrite = [options valueForKey:@"overwrite"] == nil ? YES : [[options valueForKey:@"overwrite"] boolValue];
             BOOL appendToExistingFile = [destPath RNFBContainsString:@"?append=true"];
+            
+            appendToExistingFile = !overwrite;
+
             // For solving #141 append response data if the file already exists
             // base on PR#139 @kejinliang
             if(appendToExistingFile)
             {
                 [fm createFileAtPath:destPath contents:[[NSData alloc] init] attributes:nil];
             }
-            writeStream = [[NSOutputStream alloc] initToFileAtPath:destPath append:YES];
+            writeStream = [[NSOutputStream alloc] initToFileAtPath:destPath append:appendToExistingFile];
             [writeStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
             [writeStream open];
         }