Spec fix/workaround (in README and in webvalve/rspec)

[smudge]

/domain @Betterment/webvalve-core @samandmoore @effron
/no-platform

It looks like #40 was merged but never released until 1.0.0 [correction: it was released but we never pulled it into our app] -- I noticed a regression in our local app due to the way our tests had been configured:

  config.around :each do |ex|
    WebValve.setup
    ex.run
  end

Unfortunately, "setup" now raises every time due to duplicate stubs.

This PR updates the rspec helper and the README to include the fix I made to the local app, but I'm wondering if this merits a slightly different API so that test-resets can be handled in a single command.

Maybe WebValve.reset should implicitly call setup again? And maybe we need a WebValve.clear that does what reset currently does and clears everything without setting up?

(Just brainstorming 🤔)

[nanda-prbot]

Needs somebody from @Betterment/webvalve-core, @samandmoore, and @effron to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[effron]

<<domainLGTM

[nanda-prbot]

Approved! 🚢 🙆‍♀️ 💥