Fix JSON segmentation bug that affected French

Fix a bug in the JSON streaming code that GoldCorpus uses. Escaped
slashes were being handled incorrectly. This bug caused low scores for
French in the early v2.1.0 alphas, because most of the data was not
being read in.

Fittingly, the document that triggered the bug was a Wikipedia article about
Perl. Parsing perl remains difficult!

spacy/gold.pyx

@@ -346,12 +346,12 @@ def _json_iterate(loc):
     cdef char close_curly = ord('}')
     for i in range(len(py_raw)):
         c = raw[i]
-        if c == backslash:
-            escape = True
-            continue
         if escape:
             escape = False
             continue
+        if c == backslash:
+            escape = True
+            continue
         if c == quote:
             inside_string = not inside_string
             continue