Don't truncate BigInts

BigInts are currently converted to JS Numbers, which can't fit values
over 2**53.

Fixes databricks#259

lib/result/ArrowResultConverter.ts

@@ -205,7 +205,7 @@ export default class ArrowResultConverter implements IResultsProvider<Array<any>
     }
 
     // Return other values as is
-    return typeof value === 'bigint' ? Number(value) : value;
+    return value;
   }
 
   private convertThriftTypes(record: Record<string, any>): any {

lib/result/utils.ts

@@ -49,12 +49,11 @@ function convertJSON(value: any, defaultValue: any): any {
 }
 
 function convertBigInt(value: any): any {
-  if (typeof value === 'bigint') {
-    return Number(value);
-  }
   if (value instanceof Int64) {
-    return value.toNumber();
+    return BigInt(value.toString());
   }
+
+  // Do not convert a BigInt away from the BigInt type
   return value;
 }