Adds support to HTTP::Params.encode encode an arrays of values for a key.

[rodrigopinto]

What is this PR for?

This PR is for adding support to HTTP::Params.encode encode a key with an array of values. Like:

data = {"key" => ["a", "b"], "foo" => "bar"}

puts HTTP::Params.encode(data) # => "key=a&key=b&foo=bar"

This closes #7826

[jwoertink]

How would slightly more complex structures look?

data = {"key" => {"subkey" => ["a", "b"], "foo" => "bar", "baz" => [["c"], ["d"]]}
puts HTTP::Params.encode(data) # => ??

Or would just say that the default lib only supports very simple structures, and all complex matters need to be handled by individual frameworks?

[oprypin]

application/x-www-form-urlencoded doesn't define any such complex structures

[oprypin]

So the thing is,
this already exists

data = {"key" => ["a", "b"], "foo" => ["bar"]}

puts HTTP::Params.new(data) # => "key=a&key=b&foo=bar"

https://carc.in/#/r/71jb

so why introduce another slightly different and inconsistent approach?

[oprypin]

The implementation here looks good though, and actually doesn't add much complexity.

[rodrigopinto]

@oprypin The problem on extending Params.new is that it expects HTTP::Params.new(raw_params : Hash(String, Array(String))) so the parameters should always be String => Array(String) and would never accept String => String.

So instead of extending the Params.new I added the behavior mentioned on the issue.

From my point of view, be too strict on only accepting values as Array, is like passing the problem to the clients that will use Params.new. I don't mind to have another implementation of the Params.new accepting HTTP::Params.new(raw_params : Hash(String, String)), but then I would change the current implementation. WDYT?

[asterite]

Some minor changes, but after that I'll approve and merge.

[yxhuvud]

In practice, one problem here is that there is no standardized way to do it, so people have invented a whole bunch of ways. See https://swagger.io/docs/specification/describing-parameters/#query-parameters for a list of a bunch of different options.

[rodrigopinto]

@asterite Applied the CR suggestions and pushed. The red on CI is something not related.