send_data should remove "Pragma" header if set to "no-cache"

In the streaming mix-in there is code to fix an issue with IE browsers when downloading a file:

# Fix a problem with IE 6.0 on opening downloaded files:
# If Cache-Control: no-cache is set (which Rails does by default),
# IE removes the file it just downloaded from its cache immediately
# after it displays the "open/save" dialog, which means that if you
# hit "open" the file isn't there anymore when the application that
# is called for handling the download is run, so let's workaround that
headers['Cache-Control'] = 'private' if headers['Cache-Control'] == 'no-cache'

It should also remove the "Pragma" header if headers['Pragma'] == 'no-cache' because the line above will not fix the issue if the Pragma header is set.

Edited by Rohit Arondekar for formatting.