properly support append-mode in RemoteFileBuffer

fs/remote.py

@@ -75,7 +75,7 @@ class RemoteFileBuffer(object):
             self._lock = fs._lock.__class__()
         else:
             self._lock = threading.RLock()
-        if "r" in mode or "+" in mode:
+        if "r" in mode or "+" in mode or "a" in mode:
             if rfile is not None:
                 if hasattr(rfile,"read"):
                     data = rfile.read(1024*256)

fs/tests/__init__.py

@@ -404,8 +404,10 @@ class FSTestCases:
         f2.close()
         self.assert_(checkcontents("b.txt", test_strings[0]))
         f3 = self.fs.open("b.txt", "ab")
+        self.assertEquals(f3.tell(),len(test_strings[0]))
         f3.write(test_strings[1])
         f3.write(test_strings[2])
+        self.assertEquals(f3.tell(),len(all_strings))
         f3.close()
         self.assert_(checkcontents("b.txt", all_strings))
         f4 = self.fs.open("b.txt", "wb")