Use PreparedRequest; print request on 500 error

Still trying to debug the intermittent failures
(Docverter/docverter#28)

docverter.py

@@ -82,14 +82,28 @@ def _process_file(source_text, to_format, from_format, extra_args):
         temp_file.write(source_text)
         temp_file.seek(0)
 
-        resp = requests.post(
-            url,
+        req = requests.Request(
+            'POST', url,
             data={'from': from_format, 'to': to_format},
             files={'input_files[]': temp_file},
         )
+        prepared = req.prepare()
+        session = requests.Session()
+        resp = session.send(prepared)
+        # import pdb; pdb.set_trace()
         if resp.ok:
             return resp.text
         else:
+            if resp.status_code == 500:
+                req = prepared
+                print('**** Got a 500 error from server *****')
+                print('{}\n{}\n{}\n\n{}'.format(
+                    '-----------START-----------',
+                    req.method + ' ' + req.url,
+                    '\n'.join('{}: {}'.format(k, v)
+                              for k, v in req.headers.items()),
+                    req.body,
+                ))
             print('temp_file = %r' % temp_file)
             print('temp_file.name = %r' % temp_file.name)
             raise RuntimeError(