Buffer_size improvements

[matemaciek]

• Expose buffer_size in response.send_file
• Adjust default in response to match the one in server
• Use server's buffer_size when auto-serving files.

WIth current architecture I don't see an easy way to bound response.send_file's buffer size to the one that exists in server.

[dhalbert]

We would appreciate comments from current users of this library -- thanks.