[dart-dio] Serializes request bodies using a specific serializer

Created by: josh-burton

Serializes request bodies using a specific serializer so a discriminator is not added

When using the generic serialize method, built value will add a descriminator to the serialized json. For APIs with strict parsing, this causes parsing errors.

By finding the specific serializer for the type built value omits the descriminator.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

CC: @ircecho @swipesight @jaumard @amondnet

added Client: Dart Enhancement: General labels

requested review from @jimschubert

changed milestone to %5.0.1

closed

mentioned in merge request !8335 (merged)

changed title from [dart-dio] Serializes request bodies using a specific serializer so a… to [dart-dio] Serializes request bodies using a specific serializer