[secrets] Explicitly set Content-Length in HTTP response.

This should prevent chunked-size encoding.

Bug: IN-418
Change-Id: Ib7ce0f0719e36dc07024a0f5d5b948b6f7daac8a
diff --git a/secrets/server.go b/secrets/server.go
index 2d30326..5723a4f 100644
--- a/secrets/server.go
+++ b/secrets/server.go
@@ -31,6 +31,7 @@
 	}
 	log.Printf("Serving secret for \"%s\"\n", testName)
 	log.Printf("SHA256 of secret: %x", sha256.Sum256([]byte(secret)))
+	w.Header().Set("Content-Length", string(len(secret)))
 	w.WriteHeader(http.StatusOK)
 	w.Write([]byte(secret))
 }