Code cleanup for ActionDispatch::Flash#call

The nested `if` was replaced by using `presence` which takes account for
the given hash when it is `nil` or when it is empty. The `else` was
removed because what it was doing was to assign to `env[KEY]` the
value it already had.
Fixing build broken by this change

c43ca06ca091fc09e2c86bb051ac92b648f12b64
Convert FlashHash in a Hash with indifferent access
Stringify the incoming hash in FlashHash

Stringify the incoming as well to handle incoming symbol keys from
marshalled sessions
Typo fixes [ci skip]
Fixed broken flash tests
Missed FlashHash#replace
Remove test that depends on future functionality
Merge pull request #55 from square/fix-the-flash

Fix the flash