Change targeted Airship server on a request basis

[romainsalles]

What does this do and why?

Users having to deal with multiple Airship servers (EU/US) on the same machine can't use this gem since you can only configure the server globally, not on a request-basis.

This PR adds this functionality by adding a new optional server parameter to the Client initializer.

Additional notes for reviewers

• No breaking change for current users of the Gem
• One of the existing specs wasn't testing anything: it's been fixed
• Please read commit by commit, otherwise it will be complicated to follow

Here is a description of the commits to help you review the PR:

1. I've added some specs on Client and PageIterator to prepare my refactoring
2. To prepare my next change, I've refactored PageIterator to improve readability
3. I've added a new next_page_path attribute to PageIterator which permits initialising the iteration with a "path" instead of an "url" (preparation for the next commits)
4. I've changed all the xxx_url methods into xxx_path
5. Small refactoring on a misleading method name
6. Finally, add the optional server parameter to the Client initializer ✌️
7. Bonus: fixed highlights of the README

Testing

• I wrote tests covering these changes
• I've tested for Ruby version 2.6.0

Airship Contribution Agreement

• I've filled out and signed Airship's contribution agreement form.