Add `optimistic_json` option on `RequestValidation`

This allows JSON to be unpacked even when the `Content-Type` is not
`application/json` because it's a common mistake to give it the wrong
value. Curl for example, will send `x-www-form-urlencoded` unless
overridden to something else. This option is off by default.

Fixes #47.
Add option to README
Merge pull request #49 from interagent/brandur-optimistic-json

Add `optimistic_json` option on `RequestValidation`