Adding Trailing Slash, Fix Double Slash Issue in `_get_url`, and Edge Case Tests#1598

WilliamBergamin merged 10 commits intoslackapi:mainslackapi/python-slack-sdk:mainfrom

HTSagara:adding-trailing-slash-with-tests-and-edge-casesHTSagara/python-slack-sdk:adding-trailing-slash-with-tests-and-edge-casesCopy head branch name to clipboard