Allow original non-updateable headers in revalidated response

index.js

@@ -467,8 +467,7 @@ module.exports = class CachePolicy {
         // of the corresponding header fields in the stored response.
         const headers = {};
         for(const k in this._resHeaders) {
-            if (excludedFromRevalidationUpdate[k]) continue;
-            headers[k] = k in response.headers ? response.headers[k] : this._resHeaders[k];
+            headers[k] = k in response.headers && !excludedFromRevalidationUpdate[k] ? response.headers[k] : this._resHeaders[k];
         }
 
         const newResponse = Object.assign({}, response, {

test/updatetest.js

@@ -75,6 +75,13 @@ describe('Update revalidated', function() {
         assert(!notModifiedResponseHeaders(simpleRequest, lastModifiedResponse, simpleRequest, cacheableResponse));
     });
 
+    it('Skips update of content-length', function(){
+        const etaggedResponseWithLenght1 = withHeaders(etaggedResponse, {'content-length':1});
+        const etaggedResponseWithLenght2 = withHeaders(etaggedResponse, {'content-length':2});
+        const headers = notModifiedResponseHeaders(simpleRequest, etaggedResponseWithLenght1, simpleRequest, etaggedResponseWithLenght2);
+        assert.equal(1, headers['content-length']);
+    });
+
     it('Ignored if validator is different', function(){
         assert(!notModifiedResponseHeaders(simpleRequest, lastModifiedResponse, simpleRequest, etaggedResponse));
         assert(!notModifiedResponseHeaders(simpleRequest, lastModifiedResponse, simpleRequest, weakTaggedResponse));