Fix missing argument, Set content_type in handle_request_mapping

[whywaita]

fixed: #70

lib.api_endpoint.api_metamodel2spec.handle_request_mapping needs content_type argument, but it's missing.

vmware-openapi-generator/lib/api_endpoint/oas3/api_metamodel2openapi.py

Lines 32 to 34 in 54cf631

	par_array, url = self.handle_request_mapping(url, http_method, service_name,
	operation_id, params, type_dict,
	structure_dict, enum_dict, show_unreleased_apis, api_open_ph)

This commit adds content_type argument.
So I can complete generating.

Signed-off-by: whywaita whywaita@whywrite.it

[goneri]

@antonOO, is it possible to get a review of this PR?