fix sent_start in serialization

spacy/tokens/doc.pyx

@@ -701,9 +701,12 @@ cdef class Doc:
                 for i in range(length):
                     if array[i, col] != 0:
                         self.vocab.morphology.assign_tag(&tokens[i], array[i, col])
-        set_children_from_heads(self.c, self.length)
+        # set flags
         self.is_parsed = bool(HEAD in attrs or DEP in attrs)
         self.is_tagged = bool(TAG in attrs or POS in attrs)
+        # if document is parsed, set children
+        if self.is_parsed:
+            set_children_from_heads(self.c, self.length)
         return self
 
     def get_lca_matrix(self):
@@ -779,7 +782,16 @@ cdef class Doc:
         RETURNS (bytes): A losslessly serialized copy of the `Doc`, including
             all annotations.
         """
-        array_head = [LENGTH, SPACY, TAG, LEMMA, HEAD, DEP, ENT_IOB, ENT_TYPE]
+        array_head = [LENGTH, SPACY, LEMMA, ENT_IOB, ENT_TYPE]
+
+        if self.is_tagged:
+            array_head.append(TAG)
+        # if doc parsed add head and dep attribute
+        if self.is_parsed:
+            array_head.extend([HEAD, DEP])
+        # otherwise add sent_start
+        else:
+            array_head.append(SENT_START)
         # Msgpack doesn't distinguish between lists and tuples, which is
         # vexing for user data. As a best guess, we *know* that within
         # keys, we must have tuples. In values we just have to hope

spacy/tokens/token.pxd

@@ -48,6 +48,8 @@ cdef class Token:
             return token.ent_iob
         elif feat_name == ENT_TYPE:
             return token.ent_type
+        elif feat_name == SENT_START:
+            return token.sent_start
         else:
             return Lexeme.get_struct_attr(token.lex, feat_name)
 
@@ -70,3 +72,5 @@ cdef class Token:
             token.ent_iob = value
         elif feat_name == ENT_TYPE:
             token.ent_type = value
+        elif feat_name == SENT_START:
+            token.sent_start = value