Issues with SendHttpRequest function in SlackApi.h

[TexasRanger-ops]

As it works with the Slack channel, this is for reference if the same function is to be used in the development of other channels. You mention in the code comments that the default request method is GET, but it appears to actually send a POST request with a 0 body due to weird handling of the NULL value passed into the data parameter. This works fine with Slack's API as they don't seem to care about the difference, but does raise issues when other API's (Microsoft for example) require it to be a GET request.

Thanks for the awesome project!

[grzryc]

👍 Good catch