Deserialize header parameters regardless of casing

[ydah]

Rack exposes X-Ids: 1,2 as X-IDS, while parameter deserialization looks up the schema name with exact casing. An OpenAPI 3 array header named X-Ids is therefore left as a string and rejected with a 400 response.

Match header names case-insensitively before deserialization. Add request-level regression coverage for array headers and a unit test for object headers, unknown headers, and the original input hash.