Encode strings before using them as URLs for Add Column by Fetching URLs - fixes #6140

[tfmorris]

Fixes #6140

Changes proposed in this pull request:

• Automatically encode URL before using it for fetch, using the correct encoding for each of the three different pieces of the URL which need encoding.

One question I have is whether we should be percent-decoding the string before feeding it to the URL encoder. This could potentially make it more compatible with existing recipes which uses .encode("url") as well as allowing the use of other sources of pre-encoded URLs, but is opening up a little bit of a can of worms.

Note also that this is based on the branch with the fix for #6137 and includes it, on the assumption that it will be reviewed and merged first.

[tfmorris]

@wetneb ping. I addressed your comment a couple of weeks ago and this is ready for re-review.