Fix parsing urlencoded pairs with empty values.

[adamroyle]

There is a bug with parsing curl statements with empty values. This fixes the bug and adjusts the tests to expected behavior.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[filfreire]

Hi @adamroyle thanks for submitting this! We're eager to help you get this into a merged state.

While I was testing this PR out locally I've found some cases where we will still not properly render urlencoded pairs. Can you provide an example you used for testing this PR?

[adamroyle]

I'm just using a simple example:

curl 'http://localhost/' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  --data-raw 'param1=&param2=&param3=value'

As an aside, however, I found another issue where Bash-style ANSI-C quoted strings (eg. --data-raw $'....') were not decoded properly. Google Chrome's "Copy as cURL" feature uses them for JSON strings that contain special chars.

eg.

curl 'http://localhost/' \
  -H 'Content-Type: application/json;charset=UTF-8' \
  --data-raw $'{"name":"O\'Reilly"}'

I spent a while trying to get it to work but ran into too many issues (mainly with general shell parsing libraries not implementing Bash-style quotes), and instead built a parser for "Copy as fetch (Node.js)" instead which has been much more reliable.