Require exactly one name for a requires or optional block

[ericproulx]

A requires or optional given a block silently declares something other than what it reads unless it names exactly one parameter.

No name. The block's params are declared one level up, and under optional they are still required:

params do
  optional type: Hash do
    requires :x
  end
end
# POST {}        => 400 "x is missing"
# POST {"x": 1}  => 201, declared {"x": 1}

Several names. Every name is required, but only the first gets the block's params, and the rest are left out of declared:

params do
  requires :a, :b, type: Hash do
    requires :x
  end
end
# POST {"a": {"x": 1}, "b": {}}        => 201, declared {"a": {"x": 1}}
# POST {"a": {"x": 1}, "b": {"x": 2}}  => 201, declared {"a": {"x": 1}}

DSL::Parameters#declare now raises ArgumentError ("a params block needs exactly one parameter name, got n") for either form.

ParamsScope#new_scope guarded its group-type check with if element, which could only be false for a block with no name. The branch was never covered, and it is how this came up. With the name now enforced, the check always runs, and the type || Array fallback after it is gone.

UPGRADING explains how to rewrite either form. Specs cover both forms for requires and optional; all four fail on master.

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