http-server-js - support optional request bodies

[chrisradek]

Related to #6561

For some tests that make a post request with an empty request body, the contentType is not being set.

Example test:

typespec/packages/http-specs/specs/parameters/body-optionality/mockapi.ts

Lines 40 to 45 in bf528cd

	uri: "/parameters/body-optionality/optional-explicit/omit",
	method: "post",
	request: {},
	response: {
	status: 204,
	},

However the current http-server-js emitted code will only run the generated handler for the above scenario if the content-type is present and set to application/json.

I'd expect that when the request body is optional, the content-type header may be optional when the body is not provided.