Fix segmentation fault when calling ray.put on a dictionary with object keys (#548)

* fix segfault when serializing dict key

* fix style

* fix test

* Fix linting.

test/runtest.py

@@ -89,8 +89,14 @@ COMPLEX_OBJECTS = [
 
 
 class Foo(object):
-  def __init__(self):
-    pass
+  def __init__(self, value=0):
+    self.value = value
+
+  def __hash__(self):
+    return hash(self.value)
+
+  def __eq__(self, other):
+    return other.value == self.value
 
 
 class Bar(object):
@@ -139,7 +145,8 @@ TUPLE_OBJECTS = [(obj,) for obj in BASE_OBJECTS]
 DICT_OBJECTS = ([{obj: obj} for obj in PRIMITIVE_OBJECTS
                  if (obj.__hash__ is not None and
                      type(obj).__module__ != "numpy")] +
-                [{0: obj} for obj in BASE_OBJECTS])
+                [{0: obj} for obj in BASE_OBJECTS] +
+                [{Foo(123): Foo(456)}])
 
 RAY_TEST_OBJECTS = BASE_OBJECTS + LIST_OBJECTS + TUPLE_OBJECTS + DICT_OBJECTS