Fix thread-safety issue in HTTP::CookieJar loading

[bradrobertson]

HTTP::CookieJar uses unsafe class caching for dynamically loading cookie jar implementations. If 2 rest-client instances are instantiated at the same time, (for instance if you spawn 2 threads), then non-deterministic behaviour can occur whereby the Hash cookie jar isn't entirely loaded and cached. This sometimes raises an exception because the Hash CookieJar can't be found (it's a race condition)

See:

• Thread-unsafe class caching sparklemotion/http-cookie#27
• undefined method 'implementation' for HTTP::CookieJar::AbstractStore:Class sparklemotion/http-cookie#6

Forcing an instantiation of the jar onload will force the Hash CookieJar to load before the system has a chance to spawn any threads.

Note this should technically be fixed in rest-client itself however that library appears to be stagnant so we're forced to fix it here. This object should get GC'd as it's not referenced by anything

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[bradrobertson]

It looks like it's just Rubocop failures on this test suite (nothing that I actually changed) Is there a standard on what you want done on PRs for this gem? Obviously my changes here aren't the reason this i failing

[nickbair-nylas]

@bradrobertson thanks for this! Don't worry about the RuboCop failures in Travis CI--that's an item that's on our plate to clean up.

[nickbair-nylas]

LGTM, thanks!