fix: Ignore Content-Length for methods without payload semantics

[0x676e67]

Tracing line: https://github.com/hyperium/hyper/blob/621d8e4d7788bfd2d62d15d40a73efae7f9a0bf0/src/proto/h2/client.rs#L664

This pull request includes several changes to the examples/headers_order.rs and src/util/client/request.rs files to improve the handling of HTTP requests and headers. The most important changes include modifying the request method in the example, updating the add_content_length_header function to consider the HTTP method, and introducing a new utility function to check method payload semantics.

Improvements to HTTP request handling:

• examples/headers_order.rs: Changed the request method from POST to GET in the example to demonstrate a different type of request.

Enhancements to header management:

• src/util/client/request.rs: Updated the body method to include the HTTP method when adding the Content-Length header, ensuring that headers are sorted correctly based on the method and order.
• src/util/client/request.rs: Modified the add_content_length_header function to accept the HTTP method as a parameter and check if the method has defined payload semantics before adding the Content-Length header.

New utility functions:

• src/util/client/request.rs: Added the method_has_defined_payload_semantics function to determine if an HTTP method has defined payload semantics, improving the accuracy of header management.
• src/util/client/request.rs: Introduced the map_alpn_protos function to map HTTP versions to ALPN protocols, enhancing protocol handling.