* Support to set header names of `ClientRequest` as Camel-Case
This is the case for supporting to request for servers which don't
perfectly implement the `RFC 7230`. It is important for an app
which uses `ClientRequest` as core part.
* Add field `upper_camel_case_headers` to `ClientRequest`.
* Add function `set_upper_camel_case_headers` to `ClientRequest`
and `ClientRequestBuilder` to set field `upper_camel_case_headers`.
* Add trait `client::writer::UpperCamelCaseHeader` for
`http::header::HeaderName`, let it can be converted to Camel-Case
then writed to buffer.
* Add test `test_client::test_upper_camel_case_headers`.
* Support upper Camel-Case headers
* [actix-http] Add field `upper_camel_case_headers` for `RequestHead`
* [actix-http] Add code for `MessageType` to support upper camel case
* [awc] Add functions for `ClientRequest` to set upper camel case
* Use `Flags::CAMEL_CASE` for upper camel case of headers
