Custom content-type header with FormData body warning is dead code

[harrishancock]

We have code which tries to warn the user when they create an HTTP message with a FormData body and a custom Content-Type header:

workerd/src/workerd/api/http.c++

Lines 180 to 184 in ee6d283

	IoContext::current().logWarning(
	"A FormData body was provided with a custom Content-Type header when constructing "
	"a Request or Response object. This will prevent the recipient of the Request or "
	"Response from being able to parse the body. Consider omitting the custom "
	"Content-Type header.");

The warning is logged when this if condition is true:

workerd/src/workerd/api/http.c++

Line 177 in ee6d283

} else if (MimeType::FORM_DATA == ct) {

The condition is guaranteed never to be true, because:

• MimeType::FORM_DATA is a ConstMimeType (with content: multipart/form-data)
• ct is a kj::String (with content: multipart/form-data; boundary=...)
• ConstMimeType::operator==(kj::StringPtr) does not parse the string, but instead splits at the first / and compares everything before, then everything after:

  workerd/src/workerd/util/mimetype.h

  Line 38 in ee6d283

  strcaseeq(subtype_, other.slice(slashPos + 1));

• "form-data" != "form-data; boundary=...", so the if condition is always false

I would say there are two bugs here:

1. ExtractedBody::contentType should preferably strongly typed, not just a kj::String
2. ConstMimeType::operator==(kj::StringPtr) is a footgun, and should probably not exist -- comparing string MIME types should involve parsing them