default collection check returns false for a _default._default collection

Concerned code:

https://github.com/couchbase/libcouchbase/blob/21284a28b333a0695ce1e221847d920d9b305422/src/instance.cc#L932

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Here, lcb returns true if the passed scope.collection is not equal to "_default".
This incorrect return value is then used for returning a "LCB_ERR_SDK_FEATURE_UNAVAILABLE" for a default collection back to the caller (here eventing).

Effect on eventing : This broke almost all of eventing's upgrade tests where a 7.0 eventing consumer (using 3.1.0 lcb) tries to run a KV operation against a 6.6.x memcached.