Enforce utf8 encoding

- Implements new middleware, `EnforceEncoding`, that encodes all param string values to `UTF-8`
- Implements new method,`enforce_utf8`,  in `Hyperdrive::Utils` 
We should add a test for `Encoding::ASCII_8BIT` (which is supplied as a constant and not a string to `String#encode`), which is the issue that prompted this change.