CORS: safelist Content-Length header

Tests: web-platform-tests/wpt#10930.

Fixes #622.

Author: benjamingr

fetch.bs

@@ -450,6 +450,7 @@ for `<code>Authorization</code>`.
 <ul class=brief>
  <li>`<code>Cache-Control</code>`
  <li>`<code>Content-Language</code>`
+ <li>`<code>Content-Length</code>`
  <li>`<code>Content-Type</code>`
  <li>`<code>Expires</code>`
  <li>`<code>Last-Modified</code>`
@@ -6238,6 +6239,7 @@ Arne Johannessen,
 Arthur Barstow,
 Axel Rauschmayer,
 Ben Kelly,
+Benjamin Gruenbaum,
 Benjamin Hawkes-Lewis,
 Bert Bos,
 Björn Höhrmann,