Make encode function safe.

[nsaunders]

What does this pull request do?

This pull request is a follow-up to #12 as well as purescript-globals#18. With a safe encodeURIComponent function available in purescript-globals as of 4.1.0, we can surface possible failure in this library's encode API by changing its type to FormURLEncoded -> Maybe String.

Where should the reviewer start?

You will find that the code changes involved are minimal.

How should this be manually tested?

I figured that verifying the example shown in the README (including my small update) was a sensible test:

encode $ fromArray [ Tuple "hey" Nothing, Tuple "Oh" $ Just "Let's go!" ]
(Just "hey&Oh=Let's%20go!")

Other Notes:

• I already updated the README, so no follow-up should be required there.
• At the risk of stating the obvious, this is a breaking API change.
• I will update Add decode function. #12 if and when this pull request is merged.

One more thing... Thanks @thomashoneyman and @garyb for your suggestions and for making this possible to implement by updating purescript-globals.

[thomashoneyman]

Thanks @nsaunders! I think it looks good -- just a couple suggestions.

Would you mind updating the README so that it mentions both encoding and decoding now, with examples? With that in place I'd be happy to merge, failing any other suggestions from @garyb.

[thomashoneyman]

Whoops -- I forgot you added that in #12. Never mind!

[nsaunders]

Great suggestion to use traverse @thomashoneyman, thanks!

[thomashoneyman]

@nsaunders I'm going to wait to see if @garyb has anything else to say about this or #12; if not, then -- if you've updated #12 to use the new safe functions -- tomorrow I'll merge both of them in

[nsaunders]

@thomashoneyman thanks - #12 has been updated as well, but, since it is rebased over this branch, you'll get a better view of the diffs if you merge this one first.

[thomashoneyman]

Thanks!