Expanding the query cache keys with limit, skip and fields
Fixing cache issue on querying all / first with same selector
Update QueryCache spec

to show need for including the "skip" value in the cache key
Without :cursor / :first in cache key spec fails

running `rspec spec/mongoid/query_cache_spec.rb`

"Mongoid::QueryCache when querying in the same collection with same
selector when querying only the first queries again"

will throw following error:

"no implicit conversion of String into Integer"
Adding :first / :cursor context to the cache key

With this change, the previous failing spec is solved.
Merge pull request #3562 from zenjoy/fix/query-cache

Expanding the query cache keys with limit, skip and fields