Python: adjusted input.read(size) argument value interpretation.

Previously, passing 0 resulted in reading the whole body and all negative
values raised an exception.

Now the behaviour is in consistentance with io.RawIOBase.read() interface,
and passing 0 returns empty (byte) string, while -1 results in reading the
whole body.

src/nxt_python_wsgi.c

@@ -1136,11 +1136,13 @@ nxt_py_input_read(nxt_py_input_t *self, PyObject *args)
                 return NULL;
             }
 
+            if (size != -1) {
                 return PyErr_Format(PyExc_ValueError,
                                   "the read body size cannot be zero or less");
             }
+        }
 
-        if (size == 0 || size > (Py_ssize_t) ctx->req->content_length) {
+        if (size == -1 || size > (Py_ssize_t) ctx->req->content_length) {
             size = ctx->req->content_length;
         }
     }

test/test_python_application.py

@@ -243,7 +243,6 @@ Connection: close
 
         self.assertEqual(self.post(body=body)['body'], body, 'input iter')
 
-    @unittest.expectedFailure
     def test_python_application_input_read_length(self):
         self.load('input_read_length')