Allow 'multi' and 'sprite' with urls, add 'download_generated_sprite' and 'download_multi' methods

[YomesInc]

Brief Summary of Changes

User can now pass ‘urls’ as a source of sprite/multi.

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:

[patrick-tolosa]

Hey,
Please add a detailed description in the PR of what was changed, and why.

This PR is very complex to be reviewed without any context.
I can see that a refactor was done around the api_download_url but I'm not sure I understand why.

[YomesInc]

Since implemented feature assumes similar changes to multiple methods, we've added build_multi_and_sprite_params similar to other SDKs, e.g. PHP.

Another part of the PR is adding download_generated_sprite and download_multi. By looking at it, all download methods have similar logic: set mode=download parameter, sign parameters, build a URL. And this logic applies not only to new download_ methods, but to download_archive_url as well. Instead of having 3 places doing same steps, we've decided to implement api_download_url.

As for tests, we've implemented a couple of helpers:

• beASprite
• beASignedDownloadUrl
• beAMulti
  This way we get a deeper check than usually done in unit tests and tests are more explicit about expectations. E.g. by expect(result).to.beASprite() we can see that we expect a sprite and an extensive set of checks makes sure that we have a sprite rather than a somewhat similar object.

That's the reasoning for the changes made. Please let us know if you'd like more specific details for any part of the changes.

[patrick-tolosa]

Thanks @YomesInc
Lets fix the conflicts and then we can review this PR.