Add header line when writing to cookies.txt

When Python is reading a Netscape-format cookies.txt file, it seems to
require an initial line that looks like this:

    # Netscape HTTP Cookie File
Remove word "Netscape" from cookiejar header line

The header line was added for compatibility with Python, but Python
only requires "HTTP Cookie File"; the word "Netscape" is optional.
Merge pull request #293 from mmorearty/patch-cookiejar-header

Add header line when writing to cookies.txt