bug: fix case where null json value for datetime fails

Json like:

{
  "assignedto": "3",
  "duedate": null,
  "files": [],
  "keyword": [],
  "messages": [
    "4",
    "13"
  ],
  "nosy": [],
  "priority": "2",
  "status": "3",
  "superseder": [],
  "title": "issue title"
}

is produced by a GET request using verbose=0. When submitted to the
rest interface results in:

  property duedate: 'None' is an invalid date (Not a date spec: 'None'
  ("yyyy-mm-dd", "mm-dd", "HH:MM", "HH:MM:SS" or "yyyy-mm-dd.HH:MM:SS.SSS"

Fix FsValue to map null to the None object rather than 'None' string.

Author: rouilj

roundup/rest.py

@@ -2782,6 +2782,8 @@ def __init__(self, name, val):
             elif isinstance(val, type([])):
                 # then lists of strings
                 self.value = [u2s(v) for v in val]
+            elif val is None:
+                self.value = None
             else:
                 # then stringify anything else (int, float)
                 self.value = str(val)