fixes output stream casting problem

faststreams/output_stream.nim

@@ -6,7 +6,9 @@ type
     buffer: string
     startOffset: int
 
-  OutputStream* = object
+  # somehow casting outputDevice to
+  # OutputStreamVar requires RootObj
+  OutputStream* = object of RootObj
     cursor*: WriteCursor
     pages: Deque[OutputPage]
     endPos: int

tests/test_output_stream.nim

@@ -150,3 +150,14 @@ suite "output stream":
     # The final outputs are the same
     check altOutput == memStream.getOutput
 
+  test "nim odd behavior":
+    # discovered in status-im/nim-snappy#2
+    proc encode(s: OutputStreamVar) =
+      check true
+
+    proc abc(s: OutputStreamVar) =
+      OutputStreamVar(s.outputDevice).encode
+      check true
+
+    var os = new OutputStream
+    os.abc()