Fix bug preventing attribution from being displayed

[lucaswoj]

A million gold stars to @tristen for noticing this 🌟 🌟 🌟 🌟 🌟 🌟 🌟 🌟 🌟 🌟 🌟 🌟

Since #2667 we have not been displaying any attribution on our maps! Fortunately this PR has not gone out in a release yet. We desperately need to tackle #1550.

This PR fixes the bug by setting an attribution property on SourceCache.

cc @anandthakker @tristen @mourner @mollymerp

Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes or additions to public APIs
• post scores for all benchmarks on your branch and master (if you made a non-trivial code change)
• do a quick sanity check on the debug page
• get a PR review 👍 / 👎

[mollymerp]

yipes! great catch @tristen !! agreed on UI tests.

talked to @scothis and he says they use selenium automated web driver to test studio UI interactions. probably worth exploring for gl-js although he mentioned it might be difficult without DOM elements to interact with.

[anandthakker]

Yikes 😳 nice save @tristen . So glad this didn't get out into a release; 👍 x 💯 on DOM/UI tests.