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

(cherry picked from commit b36b0a3765bcacb4dcdbf12060e9e99711855da8)

Co-authored-by: ValeriyaSinevich <valeriya.sinevich@phystech.edu>
diff --git a/Lib/http/server.py b/Lib/http/server.py
index ea0e295..ca2dd50 100644
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -474,7 +474,7 @@
             })
             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: