feat: Send form allows overriding of CONTENT_TYPE

[0x676e67]

This allows overriding the default Content-Type set by the form method, similar to how the json method handles it. On some servers, we may need to use application/x-www-form-urlencoded;charset=utf-8 to replace the default application/x-www-form-urlencoded.

• json

reqwest/src/async_impl/request.rs

Line 446 in 3099f30

if !req.headers().contains_key(CONTENT_TYPE) {