Use the default http client instead of creating a new one every time

[hussachai]

This change is very simple. Just one line. The current behavior of slack.New is that it creates a new http.Client{} every single time, and that's not a good thing because http.Client should be reused. It's true that we can override http.Client by using slack.OptionHTTPClient(). The problem is that the API still creates a new http.Client instance just to be discarded right after. That's a wasteful allocation. This PR is to get rid of that while everything should remain the same.

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