Typescript+Axios: multipart/form-data correctly handled

Created by: mvniekerk

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10)

Description of the PR

When doing a multipart/formdata request, the current state is that it changes the content type to ''application/x-www-form-urlencoded" (violating the contract) and then does a toString() on the request data.

What this PR does is to use FormData as the data, it appends (instead of doing a .set()) and then omits the toString() when the content-type is multipart/form-data