pub const EXPECT: HeaderName;
Indicates expectations that need to be fulfilled by the server in order to properly handle the request.
The only expectation defined in the specification is Expect: 100-continue, to which the server shall respond with:
100 if the information contained in the header is sufficient to cause an immediate success,
417 (Expectation Failed) if it cannot meet the expectation; or any other 4xx status otherwise.
For example, the server may reject a request if its Content-Length is too large.
No common browsers send the Expect header, but some other clients such as cURL do so by default.