bugfix: mark committed to false only when it is nil in conn

[taoky]

When debugging the conn limiter today, I tried with this code in development env, to help me get conn counter value in HTTP header:

local delay, err = limit_traffic.combine(limiters, keys, states)
if not delay then
    -- ...
else
    for i, limiter in ipairs(limiters) do
        local delay, err = limiter:incoming(keys[i], false)
        local limiter_name = limiters_name[i]
        ngx.header[ "X-RateLimit-" .. limiter_name ] = err
        ngx.header[ "X-RateLimit-" .. limiter_name .. "-Key" ] = keys[i]
    end
end

for i, limiter in ipairs(limiters) do
    if limiter.is_committed then
        if limiter:is_committed() then
            ngx.log(ngx.WARN, limiters_name[i])
            table.insert(limiters_to_leave, limiter)
            table.insert(keys_to_leave , keys[i])
        end
    end
end

And surprised by that, after I added the limiter:incoming(keys[i], false) here, the limiter:is_committed() returns false even when it is actually committed.

In conn.md, it says:

* `commit` is a boolean value. If set to `true`, the object will actually record the event
in the shm zone backing the current object; otherwise it would just be a "dry run" (which is the default).

But it is actually not just "dry run" -- incoming would always set committed to false at first not matter what.

function _M.incoming(self, key, commit)
    local dict = self.dict
    local max = self.max

    self.committed = false
    -- ...
end

This PR tries to fix the lua code to align with docs, and adds a testcase for that.

[zhuizhuhaomeng]

The changes do not follow the document.
https://github.com/openresty/lua-resty-limit-traffic/pull/80/files#diff-aa64cd78441de2bae2f552bca126b6cedde9a9fa7ba046d689386cbffecc5795R229

[taoky]

Well, I doubt if this change makes sense for is_committed() docs:

- Returns `true` if the previous [incoming](#incoming) call actually commits the event
+ Returns `true` if the previous [incoming](#incoming) calls actually have committed the event