Make the helpful tolerantBodyParser utility function public

`tolerantBodyParser` is used for 3 different BodyParsers (JSON, XML, Forms) in the PlayFramework, and looks like a generally useful utility function (eg like `when`).

My own desire to use it stems from wanting to write a payload-HMAC-validating BodyParser for a GitHub webhook endpoint:

https://developer.github.com/webhooks/#delivery-headers

I want the JSON body of the content, but also to validate the binary data has the correct HMAC signature. A custom BodyParser seems to be the neatest way to do this, something like this:

```
def tolerantSignedGithubJson(maxLength: Int): BodyParser[JsValue] =
  tolerantBodyParser[JsValue]("json", maxLength, "Invalid Json") { (request, bytes) =>
    assertSecureEquals(request.headers("X-Hub-Signature"), sign(bytes, sharedSecret))
    Json.parse(bytes)
  }
```

...which I can't do if `tolerantBodyParser()` is private. The body of `tolerantBodyParser()` is sufficiently large & dense that copy-&-pasting it seems a shame.