This makes the tests much less brittle, by not depending on the exact
order of the HTTP headers, nor always requiring to check for every
single header.
- The `Error`'s kind is a now a set of variants depending on the context
of when an error could occur.
- If another error was the cause, it is now always the `source`.
Along with the `is_*` methods, this should help in understanding *when*
a certain error occurred. For example, an error setting the TLS
certificates will return a builder error, with the TLS error as the
source. This should help differentiate from a TLS error that happens
when connecting to a server.
It also makes the internal code less dependent on all the exact
dependencies that can be enabled or disabled.
- Converted `Response::text` and `Response::json` to `async fn`
- Added `Response::bytes` async fn as a counterpat to `text`.
- Added `Response::chunk` async fn to stream chunks of the response body.
- Added `From<Response> for Body` to allow piping a response as a request body.
- Removed `Decoder` from public API
- Removed body accessor methods from `Response`
- Removed `Chunk` type, replaced with `bytes::Bytes`.
- Removed public `impl Stream for Body`.
