Allow direct conversion for Document -> Response

[Alch-Emi]

This add From<Borrow<Document>> to Response, allowing users to call .into() on any reference to a Document. This means that when chaining methods together to build a document, .into() can simply be chained along. So this:

let doc = Document::new();
doc.add_text("hello")
   .add_text("world");
let resp = Response::document(doc);

can become this:

let resp = Document::new()
    .add_text("hello")
    .add_text("wello")
    .into();

This does also affect the Response::document method, but only to make it more general, so this isn't a breaking change.

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This change is 

[panicbit]

Thanks!