GLib 2.76 emits a warning if `g_string_free (_, FALSE)` is used without
using its return value, as it typically signifies a leak:
```
warning: ignoring return value of ‘g_string_free_and_steal’ declared
with attribute ‘warn_unused_result’ [-Wunused-result]
```

In this case, the libsoup example and test code did *not* leak because
it had already stolen the string data from `GString`. That’s a bit of a
layering violation (the string data briefly has two owners), so
rearrange the code to fix that and silence the warning.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>