Finish moving Content-Type onto Body

[nielsenko]

Relic keeps Content-Type on Body (Body.bodyType), not in Headers. The direction is right: the type describes the bytes, a response cannot send a type that disagrees with its body, and copyWith(body: ...) keeps the two together. Working on form and multipart parsing (#380) turned up gaps that make the model leak.

Gaps

1. Parameters were missing. MimeType and BodyType had no media type parameters, so there was nowhere to put a boundary. Forms added ContentTypeHeader to work around it, and multipart/byteranges responses from StaticHandler went out over dart:io without a boundary, so clients could not split them. The forms PR adds BodyType.parameters and fixes the byteranges case.
2. encoding does not say what was sent. Body.fromDataStream and Body.fromData default text types to UTF-8. On a request, req.encoding is utf8 both when the client sent charset=utf-8 and when it sent no charset, so code cannot apply its own default.
3. Too many types for one concept. MimeType, BodyType and ContentTypeHeader, plus io.ContentType in the dart:io adapter. Each has its own parse and encode path.
4. Two sources of truth on requests. The adapter copies the raw content-type header into Headers and also into Body.bodyType. After copyWith(body: ...), headers.contentType and body.bodyType can disagree.
5. Silent override on responses. Setting headers['content-type'] on a response is dropped by the adapter with no error. HttpResponseExtension.applyHeaders always writes Content-Type from the body.

Proposal

• Make BodyType the single media type: add BodyType.parse, keep the raw charset name next to encoding, and keep the parameters. ContentTypeHeader can then go.
• Give the request Content-Type one owner. Either the adapter leaves content-type out of Headers, or headers.contentType reads from the body.
• Fail loudly when a response carries a raw content-type header, for example an assert in debug mode.

Some of this is breaking for 2.0 users (req.encoding semantics, a raw header that used to be ignored now asserting).