feat: support multiple headers

[medz]

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[brianquinlan]

Hi @medz, is this ready for review?

[medz]

@brianquinlan I'm ready, but there are a few problems I can't solve. I observed that the mock client uses isolate, and my headers implementation cannot send messages. I don't have a good solution for the time being.

[medz]

In addition, similar to here, I think it is completely possible to use Object? to set headers, because headers are made according to MDN standards, so in theory it can allow many structures to be converted into headers

[brianquinlan]

You are changing the public interface to several widely-used interfaces, which would be massively breaking.

Multiple header values are supported by separating them my commas and assigning them to the name field e.g.

headers = {"fruit": "apple,banana"};

Maybe it would be more practical to add a function to split and combine comma-separated field values e.g.

List<String> splitHeaderValues(String values);
joinHeaderValues(Iterable<String> values);

...but now that I think about it, those functions would be trivial.

Maybe the best solution would be to document? Like at https://pub.dev/documentation/http/latest/http/BaseRequest/headers.html

[medz]

You are changing the public interface to several widely-used interfaces, which would be massively breaking.

Multiple header values are supported by separating them my commas and assigning them to the name field e.g.

headers = {"fruit": "apple,banana"};

Maybe it would be more practical to add a function to split and combine comma-separated field values e.g.

List<String> splitHeaderValues(String values);
joinHeaderValues(Iterable<String> values);

...but now that I think about it, those functions would be trivial.

Maybe the best solution would be to document? Like at https://pub.dev/documentation/http/latest/http/BaseRequest/headers.html

This PR will indeed cause breaking changes, but it will make subsequent releases permanent. Splitting using , is indeed possible, but many http header values contain , which can easily cause wrong splitting.

Maybe changing the type of e.g. http.post(headers: xxx) to Object? would be a good choice, and content built with previous versions can be migrated to the new headers implementation without changing the value type. I will make a commit later to implement it

[medz]

The PR close, new #1106 replace this