HTTP: fixed status line format for unknown status codes.

According to Section #3.1.2 of RFC 7230, after the status code
there must be a space even if the reason phrase is empty.

Also, only 3 digits allowed.

This closes #507 issue on GitHub.

docs/changes.xml

@@ -9,6 +9,12 @@
          date="" time="18:00:00 +0300"
          packager="Andrei Belov <defan@nginx.com>">
 
+<change type="bugfix">
+<para>
+invalid HTTP responses were generated for some unusual status codes.
+</para>
+</change>
+
 </changes>