Fix send_file to work with non-ascii filenames

This commit implements https://tools.ietf.org/html/rfc2231#section-4 in
order to support sending unicode characters. Tested on both Firefox and
Chromium under Linux.

This adds unidecode as a dependency, which might be relaxed by using
.encode('latin-1', 'ignore') but wouldn't be as useful.

Also, added a test for the correct headers to be added.

Previously, using a filename parameter to send_file with unicode characters, it
failed with the next error since HTTP headers don't allow non latin-1 characters.
Error on request:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/werkzeug/serving.py", line 193, in run_wsgi
    execute(self.server.app)
  File "/usr/lib/python3.6/site-packages/werkzeug/serving.py", line 186, in execute
    write(b'')
  File "/usr/lib/python3.6/site-packages/werkzeug/serving.py", line 152, in write
    self.send_header(key, value)
  File "/usr/lib64/python3.6/http/server.py", line 509, in send_header
    ("%s: %s\r\n" % (keyword, value)).encode('latin-1', 'strict'))
UnicodeEncodeError: 'latin-1' codec can't encode character '\uff0f' in position 58: ordinal not in range(256)

Fixes #1286

tests/test_helpers.py

@@ -560,6 +560,17 @@ class TestSendfile(object):
             assert options['filename'] == 'index.txt'
             rv.close()
 
+    def test_attachment_with_utf8_filename(self):
+        app = flask.Flask(__name__)
+        with app.test_request_context():
+            with open(os.path.join(app.root_path, 'static/index.html')) as f:
+                rv = flask.send_file(f, as_attachment=True,
+                                     attachment_filename='Ñandú／pingüino.txt')
+                value, options = \
+                    parse_options_header(rv.headers['Content-Disposition'])
+                assert options == {'filename': 'Nandu/pinguino.txt', 'filename*': "UTF-8''%C3%91and%C3%BA%EF%BC%8Fping%C3%BCino.txt"}
+                rv.close()
+
     def test_static_file(self):
         app = flask.Flask(__name__)
         # default cache timeout is 12 hours