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

setup.py

@@ -75,6 +75,7 @@ setup(
         'Jinja2>=2.4',
         'itsdangerous>=0.21',
         'click>=2.0',
+        'unidecode',
     ],
     classifiers=[
         'Development Status :: 4 - Beta',