Handle case sensitivity in headers for Spring Boot apps

[StalkersGlue]

Describe the bug
When defining headers on endpoints you can use upper casing. Spring by default lowercases headers. The emitted wirespec code will verify against the lowercased request header and it will fail.

To Reproduce
Steps to reproduce the behavior:

1. Have a wirespec endpoint with #{Content-Type: String}
2. Implement Spring boot app with @EnableWirespecController
3. Call endpoint with header and see stacktrace.

Expected behavior
According to RFC 7230, HTTP header names are case-insensitive. This means Content-Type, content-type, CONTENT-TYPE, and Content-type are all equivalent.