Fix UiServer.getPosted hanging in some circumstances

fixes #198

while it's not exactly clear what causes the difference in behaviour,
but under certain conditions UiServer.getPosted used to hang trying
to readline() POST request (e.g. from UiPassword login). using
read(CONTENT_LENGTH) seems to fix the issue

src/Ui/UiRequest.py

@@ -231,8 +231,12 @@ class UiRequest(object):
     # Return: <dict> Posted variables
     def getPosted(self):
         if self.env['REQUEST_METHOD'] == "POST":
+            try:
+                content_length = int(self.env.get('CONTENT_LENGTH', 0))
+            except ValueError:
+                content_length = 0
             return dict(urllib.parse.parse_qsl(
-                self.env['wsgi.input'].readline().decode()
+                self.env['wsgi.input'].read(content_length).decode()
             ))
         else:
             return {}