Sanitize rack body on POST/PUT 

Closes #7

1. if request is POST/PUT 
2. and content type is one we sanitize (SANITIZABLE_CONTENT_TYPES)
3. and there is rack.input
4. read out, sanitize, and close contents of rack.input
5. replace env['rack.input'] with original and sanitized forms wrapped in SanitizedRackInput object

Thus, we maintain a reference to the unmodified original rack.input and provide a universal interface for the sanitized input

To consider:

- We may want to consider adding the original rack input to the env, similar to how method override does
- We may want to make use of any charset included in the content-type