Add support for CORS in http requests

[Kilowhisky]

Please do not open a pull request without first filing an issue and/or discussing the feature directly with the project maintainer.

Please ensure you adhere to every item in this list

• [ ] This PR was pre-approved by the project maintainer
• [x] I have self-reviewed the code
• [x] I have added all necessary tests

Describe your changes

Adds support for OPTIONS HTTP request and returns necessary headers to allow CORS to function properly

Issue number and link

https://github.com/tidwall/tile38/issues/737

I'm not quite sure how testing should be performed here as I don't see any testing of the http interface right now.. maybe i'm missing something. Could use a pointer on where and how you'd like me to write tests.

Tests can be done with standard HTTP requests.

[Kilowhisky]

Tests added

[Kilowhisky]

Verified against my test project in chrome. Request works successfully.