tests: store ETag when using --headeronly

Previously, --headeronly would prevent --twice from working
because the ETag wasn't stored when --headeronly was used.
This feels like a bug. That feeling is reaffirmed by the fact
that this change doesn't regress any tests.

tests/get-with-headers.py

@@ -78,8 +78,8 b' def request(host, path, show):'
         else:
             sys.stdout.write(data)
 
-        if twice and response.getheader('ETag', None):
-            tag = response.getheader('ETag')
+    if twice and response.getheader('ETag', None):
+        tag = response.getheader('ETag')
 
     return response.status