Avoid TypeError when params contain a key without a value on Ruby < 2.4

[segiddins]

Fixes #1514

[segiddins]

This ought to go green when rebased atop #1513

[namusyaka]

Thanks for the pull request, this looks good to me with a nit.

[namusyaka]

You don't need to update CHANGELOG.md

[segiddins]

The contributing.md said it made it easier to do releases when PRs included changelog entries 😉

[namusyaka]

The changelog entry will automatically be generated by the release script: #1515
contributing.md will also be updated at the same time.

[segiddins]

https://github.com/sinatra/sinatra/blob/master/CONTRIBUTING.md#have-a-patch #4 says to update the changelog, I was just following the instructions 🤷

[namusyaka]

@segiddins Yes, I apologize if confusing.
But as I said earlier, the contents of contributing.md will be updated soon.
Do you want to keep your own changelog entry so much?
If not, please remove the entry from changelog.

[junaruga]

This PR is not #1516 but #1517. That confused me a little :)

[#1516](https://github.com/sinatra/sinatra/pull/1516)

[segiddins]

agh, I was trying to figure out which PR it would be before opening it

[RaVbaker]

Just realized it doesn't cover one other case. When the parameter is true Because in Ruby <=2.3 also making a dup on true is prohibited. While this PR will cover the false and nil cases. Yet not the true.

> true.dup
TypeError: can't dup TrueClass

while in >=2.4

> true.dup
=> true

And yet in JSON body payloads there can be a true, false or null/nil. Which are not dup-able in older Rubies. @segiddins @namusyaka

[dentarg]

@RaVbaker do you have a repro for the issue? I think it is better to open up a new issue than commenting here

[namusyaka]

@RaVbaker Good catch, I'll work on the issue soon