Changes to WEBSERVICE

[oleibman]

This PR started because the documentation on how to configure an Http client was out of date. However, as I investigated further, I found problems. GuzzleHttp, a client mentioned in the documentation, for example - it probably works fine when executed from a browser, but it does not work for https: requests from the Windows command line. See guzzle/guzzle#2601, where a user notes that Guzzle's own program to demonstrate how to use it doesn't work, a result that I can confirm is still true. A linked explanation says that the solution is to explicitly specify a path to a CA bundle. For starters, this is impractical from our perspective. One possible solution is to change a php.ini option which is not needed for any other purpose, and which probably needs to change frequently - a burden on users who follow that route. An alternative solution is to use a method request to specify the path to the certificate store; this also will need to change from time to time, and, worse, the only method defined in ClientInterface is sendRequest, so using this solution isn't client-agnostic, which is a stated goal of 7cb4884. Additionally, it is not clear why an external interface is needed rather than a call to file_get_contents, used elsewhere in PhpSpreadsheet, and not requiring a path to a certificate store.

I also believe that automatically evaluating WEBSERVICE for any arbitrary argument is not a good idea. I am adding a domain whitelist which the user must populate. For domains not in the whitelist, the calculation will revert to oldCalculatedValue, which is good enough for pass-through purposes, which probably encompasses most cases. That is how Excel behaves by default - it disables WEBSERVICE calls when it opens a spreadsheet which contains them. For cases where the user adds a new WEBSERVICE call, there is a choice of whitelisting the domain, or getting the result in some other way and using setCalculatedValue to store it.

Finally, when a WEBSERVICE call is evaluated, it will now accept a cell-address argument rather than just a literal string as is now the case.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary