Release `package:http` v2.0.0 with desired breaking changes

[brianquinlan]

Now that aborting requests is supported (the most requested feature for package:http), let's take the time to reorganize APIs that were implemented as mixins, extensions or subclasses (to avoid breaking changes) into their base classes.

• Move headersSplitValues into BaseClient
• Fold Abortable into BaseRequest (make sure to update the example in the README)
• Fold BaseResponseWithUrl into BaseResponse

There are some other semantic changes that we should make:

• Drop BaseRequest.send() method
• Add better feedback around the types for body argument
• Only set the charset when the body is set for certain mime types

[TekExplorer]

I'd also change MultipartFile to not include the field, and MultipartRequest.files should become a Map<String, MultipartFile>

Note that MultipartFile does not actually use the field itself - its just used by MultipartRequest.

[EchoEllet]

Drop BaseRequest.send() method

I have a suggestion that might be appropriate for v2.0.0: Throw a StateError instead of ClientException when sending an HTTP request after the client has already been closed, as this is a programming bug that typically gets fixed rather than handled at runtime.

The current code:

// pkgs/http/lib/src/io_client.dart

/// Sends an HTTP request and asynchronously returns the response.
  @override
  Future<IOStreamedResponse> send(BaseRequest request) async {
    if (_inner == null) {
      throw ClientException(
          'HTTP request failed. Client is already closed.', request.url);
    }
}

(More details in #1948 (comment))