Initial commit
Basic cache backend
Support caching falsey values
Raise ArgumentError when no block is given to Cache#cache
Add Cache#fetch
Add Cache#has_key?
Test that multiple keys can be stored
Freeze cached values to prevent mutating the cache

Data is best served cold.
Add Cache#clear!
Add Rack middleware for encapsulating the cache
Ensure cache is cleared when the app raises an error
Add delegation method for #clear! to RequestCache
Make middleware thread safe
Loosen error message test for JRuby

JRuby has a slightly different message that failed the more exact
comparison.
Add usage to readme
Add root file so Bundler's default require name works