core: Handle ‘etag’ and ‘kind’ members in JSON feeds

Set the GDataFeed:etag property and ignore the kind.
core: Don’t unset refresh token on refreshing OAuth 2.0 authorisation

When refreshing an OAuth 2.0 authorisation, only a new access token is
returned by the server — the refresh token is left unchanged, so a new
one is not returned. Do not use that as an excuse to forget the existing
refresh token.
tasks: Handle an additional form of AUTHENTICATION_REQUIRED error

Apparently there are two ‘reason’ codes which correspond to this.
tasks: Add preconditions for tasklist IDs being set when inserting tasks

Can’t insert a task to a non-tasklist.
tests: Add networked tests for the Google Tasks service

This brings test coverage of the gdata/services/tasks/ directory up to
84% of lines and 90% of functions, which is good enough.

https://bugzilla.gnome.org/show_bug.cgi?id=657539