Add written bytes count to write APIs

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

                     String path = args[0];
                     String encoding = args[1];
                     String data = args[2];
+                    int written = 0;
                     File f = new File(path);
                     File dir = f.getParentFile();
                     if(!dir.exists())
                         FileInputStream fin = new FileInputStream(src);
                         byte [] buffer = new byte [10240];
                         int read = fin.read(buffer);
+                        written = read;
                         while(read > 0) {
                             fout.write(buffer, 0, read);
                             read = fin.read(buffer);
+                            written += read;
                         }
                         fin.close();
                     }
-                    else
-                        fout.write(stringToBytes(data, encoding));
+                    else {
+                        byte[] bytes = stringToBytes(data, encoding);
+                        fout.write(bytes);
+                        written = bytes.length;
+                    }
                     fout.close();
-                    promise.resolve(Arguments.createArray());
+                    promise.resolve(written);
                 } catch (Exception e) {
                     promise.reject("RNFetchBlob writeFileError", e.getLocalizedMessage());
                 }
                     }
                     os.write(bytes);
                     os.close();
-                    promise.resolve(null);
+                    promise.resolve(data.size());
                 } catch (Exception e) {
                     promise.reject("RNFetchBlob writeFileError", e.getLocalizedMessage());
                 }

src/ios/RNFetchBlobFS.m

     }
 }
 
-+ (void) writeFileFromFile:(NSString *)src toFile:(NSString *)dest append:(BOOL)append
++ (NSNumber *) writeFileFromFile:(NSString *)src toFile:(NSString *)dest append:(BOOL)append
 {
     NSInputStream * is = [[NSInputStream alloc] initWithFileAtPath:src];
     NSOutputStream * os = [[NSOutputStream alloc] initToFileAtPath:dest append:append];
     [is open];
     [os open];
     uint8_t buffer[10240];
+    long written = 0;
     int read = [is read:buffer maxLength:10240];
+    written += read;
     while(read > 0) {
         [os write:buffer maxLength:read];
         read = [is read:buffer maxLength:10240];
+        written += read;
     }
     [os close];
     [is close];
+    return [NSNumber numberWithLong:written];
 }
 
 + (void) writeFile:(NSString *)path encoding:(NSString *)encoding data:(NSString *)data append:(BOOL)append resolver:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject
             content = [[NSData alloc] initWithBase64EncodedString:data options:0];
         }
         else if([encoding isEqualToString:@"uri"]) {
-            [[self class] writeFileFromFile:data toFile:path append:append];
-            resolve([NSNull null]);
+            NSNumber* size = [[self class] writeFileFromFile:data toFile:path append:append];
+            resolve(size);
             return;
         }
         else {
             [content writeToFile:path atomically:YES];
         }
         fm = nil;
-        resolve([NSNull null]);
+        
+        resolve([NSNumber numberWithInteger:[content length]]);
     }
     @catch (NSException * e)
     {
         }
         free(bytes);
         fm = nil;
-        resolve([NSNull null]);
+        resolve([NSNumber numberWithInteger: data.count]);
     }
     @catch (NSException * e)
     {