bpo-33663: Convert content length to string before putting to header (GH-7754)

(cherry picked from commit b36b0a3)

Co-authored-by: ValeriyaSinevich <valeriya.sinevich@phystech.edu>

Author: miss-islington

Lib/http/server.py

@@ -466,7 +466,7 @@ def send_error(self, code, message=None, explain=None):
             })
             body = content.encode('UTF-8', 'replace')
             self.send_header("Content-Type", self.error_content_type)
-            self.send_header('Content-Length', int(len(body)))
+            self.send_header('Content-Length', str(len(body)))
         self.end_headers()
 
         if self.command != 'HEAD' and body:

Misc/NEWS.d/next/Library/2018-06-17-10-48-03.bpo-33663.sUuGmq.rst

@@ -0,0 +1 @@
+Convert content length to string before putting to header.