add BigInt test

Author: belous-dp

test/write.jl

@@ -198,9 +198,11 @@ Base.string(x::AF) = string(x.f)
         ),
         # 1151
         (
-            [(a=Int128(-170141183460469231731687303715884105728),)],
+            [(a=Int128(-170141183460469231731687303715884105728),
+              b=big(-170141183460469231731687303715884105728),
+              c=big(-170141183460469231731687303715884105729),)],
             NamedTuple(),
-            "a\n-170141183460469231731687303715884105728\n"
+            "a,b,c\n-170141183460469231731687303715884105728,-170141183460469231731687303715884105728,-170141183460469231731687303715884105729\n"
         )
     ]