[typescript-rxjs] refactor to arrow functions and short hand object creation

Created by: denyo

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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
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Description of the PR

• refactor to arrow functions where possible
• abstract error throwing null checks into a function throwIfRequired
• remove unused parameter in RequiredError (this is a breaking change if people are using it outside the generated code)
• refactor headers and query objects to be created inline to be less verbose and skip redundant null checks

This all results in a noticeable code savings. In my current react project with CRA2 (9 controllers and 14 endpoints) I get a build with 4368 characters less which is about 4%, although file size after gzip it is only 230 Bytes which is about 1%.

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