add validation for baseurl

[dafinoer]

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest develop to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I am adding
• I have updated the documentation (if necessary)
• I have run the tests and they pass

This merge request fixes / refers to the following issues: ...

Pull Request Description

Just check validation for valid URL in option.baseUrl
#1373
...

[AlexV525]

Hi @dafinoer. We've made our hardfork repo public and published a new version of dio, named diox.
The new package contains the PR of the fix.
Please refer to https://pub.dev/packages/diox/versions/5.0.0-dev.1 to use the fork.
You can also see why we're working for a hardfork at cfug/diox#29 and #1607.