Failing tests for long post data

There seems to be a problem with long form data being `POST`ed. After a certain length, the value of the every byte in Request.raw_form.["name"] is set to 0.

I'm not been able to track down a root cause yet. I hit it while trying to implement persona authentication, but as you can see from the failing tests included it doesn't seem due to the content of the assertions that persona uses - even a long enough string of a single character will trigger the bug.

I'm not suggesting that these changes are merged as is, but hopefully the failing tests will be helpful in tracking things down.

Of the tests below, `"POST form data makes round trip"` passes but both `"POST long data"` and `"POST persona assertion"` fail.