Allow non latin-1 filename in FileResponse

[em92]

Fixes #790

[tomchristie]

This looks pretty reasonable yup.

Any links to other implementations (eg. Flask) that handle this case, so we can double check what they do? Any particular RFC information that's worth us following here?

[em92]

from flask import Flask, send_file
app = Flask(__name__)

@app.route('/')
def hello_world():
    return send_file("你好.txt", as_attachment=True)

https://github.com/pallets/flask/blob/64ba43411fbfb2e435455b0df2453d7809b7c5a8/src/flask/helpers.py#L614-L626