The command `gnus-summary-toggle-header' correctly shows all headers of
an article in an Emacs bug report ephemeral group, but incorrectly fails
to rehide the headers that are hidden by default.  To reproduce:

0. emacs -Q
1. C-h f gnus-re TAB (to load Gnus, making the next command available).
2. M-x gnus-read-ephemeral-emacs-bug-group RET 27008 RET (or any bug
   number).
3. SPC (to display the first article, which hides some headers).
4. t (invokes `gnus-summary-toggle-header', displaying all headers of
   the article).
5. Typing `t' again should now rehide the headers that were not shown in
   step 3, but
=> nothing happens, i.e., all headers remain visible.

The problem seems to be that after step 4, the first line of the
*Article* buffer contains a header like this:

From unknown Fri May 26 08:24:49 2017

and gnus-summary-toggle-header tests for hidden headers by looking for a
colon before the first space in that line; since the test fails, it
treats the headers as hidden, so it doesn't hide them again.

This problem also happens with other "foreign" Gnus groups; I didn't
test all types but I see it e.g. with directory groups (`G d' in the
Gnus *Group* buffer) and the Gnus help group (`G h').  If in all such
groups the first header line is like the above but the second header
line satisfies the test (it does in the cases I looked at, but I don't
know if that's always so), then the patch below is probably the simplest
fix.

In GNU Emacs 26.0.50 (build 24, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-05-24 built on rosalinde
Repository revision: bba9917299e3628e40462a762f2a14bb8df193f0
Windowing system distributor 'The X.Org Foundation', version 11.0.11901000