[TypeScript-Node] support setting the content-type header per-call

[silasbw]

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Description of the PR

This approach is inspired by the typescript-fetch implementation in swagger-codegen.

Fixes #1867

[silasbw]

CC @topce @wing328

[wing328]

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10)

[silasbw]

@macjohnny applied your suggestions, ty.

About naming .headerParams vs .headers, one argument for using .headers is that a few popular libraries/APIs use the name headers for a similar purpose (Fetch API, request, axios). Using .headers instead of .headerParams might be "the least surprising".

[macjohnny]

@silasbw ok, using .headers sounds reasonable, too.

[wing328]

@silasbw thanks for the PR, which has been merged into master.

Is it correct to say that this is a breaking change (without fallback) as I saw the changes in the method signature?

[silasbw]

I don't think it's breaking, because the change includes a default value for argument it adds to the function signature..

[wing328]

@silasbw 👌 thanks for the explanation.