SecurityManager: remove double quotes from session cookie content
SecurityManager: remove double quotes from session cookie content

Sometimes, RFC 6265-compliant [0] web servers may send back a cookie whose value
is enclosed in double quotes, such as:

    Set-Cookie: sessionCookie="s.37cf5299fbf981e14121fba3a588c02b,s.2b21517bf50729d8130ab85736a11346"; Version=1; Path=/; Domain=localhost; Discard

Where the double quotes at the start and the end of the header value are just
delimiters. This is perfectly legal: Etherpad parsing logic should cope with
that, and remove the quotes early in the request phase.

Somehow, this does not happen, and in such cases the actual value that
sessionCookie ends up having is:

    sessionCookie = '"s.37cf5299fbf981e14121fba3a588c02b,s.2b21517bf50729d8130ab85736a11346"'

As quick measure, let's strip the double quotes (when present).
Note that here we are being minimal, limiting ourselves to just removing quotes
at the start and the end of the string.

Fixes #3819.
Also, see #3820.


[0] https://tools.ietf.org/html/rfc6265

ether/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite