OAuth1.0a: don't modify HTTP response while parsing it
OAuth1.0a: don't modify HTTP response while parsing it

The QMap::operator[] modifies the QMap by inserting the key if it
doesn't find it. This makes it so that the following checks on
QMap::contains() always return true.
To correct this, we use QMap::value() and for additional safety we set
the QMap to const.

plotters/accounts-sso.signon-plugin-oauth2,plotters/accounts-sso.signon-plugin-oauth2