Add before send option

[kjarmicki-cloudinary]

Brief Summary of Changes

I'd like to be able to do adjustments to the request before it's sent to an API - things like adding custom headers etc. To achieve that I added an optional function BeforeSend that will be called right before a request is sent.

What does this PR address?

• GitHub issue (Add reference - #XX)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all the tests pass.

[kjarmicki-cloudinary]

Will use custom Transport instead