Adding POST style parameters for DELETE request.

Hi,

Grape used to accept POST style parameters for DELETE request. So I'm adding it back here.

However, I know that the HTTP spec doesn't explicitly disallow request bodies (where POST parameters go) for DELETE requests and many server implementations ignore it and you may run into issues with proxies dropping the content. So I'll leave it to you to decide if you which too add it.

References:
http://www.quora.com/Is-there-any-way-to-pass-parameters-for-HTTP-DELETE-method
http://stackoverflow.com/questions/299628/is-an-entity-body-allowed-for-an-http-delete-request