[Spec] Add default http scheme for server URL, Fix #181

Created by: grokify

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\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

fix https://github.com/OpenAPITools/openapi-generator/issues/181

1. Includes additional tests for sanitizeUrl.
2. Checks against scheme patterns shown by IANA list, which includes [0-9a-z.+\-]. Can be made more generic to match [^\p{Space}:/]
3. Ran the bin/go-petstore.sh after removing the scheme from the spec and it works as expected. Didn't check in the changes because it requires a change in the spec.

Current Behavior before PR for bin/go-petstore.sh

Removing the scheme from the spec results in the following warning and a non-working client.

[main] WARN  o.o.codegen.utils.URLPathUtils - Not valid URL: petstore.swagger.io:80/v2. Default to http://localhost.

Generated configuration.go causes mvn integration-test to fail:

func NewConfiguration() *Configuration {
	cfg := &Configuration{
		BasePath:      "petstore.swagger.io:80/v2",
		DefaultHeader: make(map[string]string),
		UserAgent:     "OpenAPI-Generator/1.0.0/go",
	}
	return cfg
}

New Behavior

bin/go-petstore.sh

[main] WARN  o.o.codegen.utils.URLPathUtils - 'scheme' not defined in the spec (2.0). Default to [http] for server URL [http://petstore.swagger.io:80/v2]

configuration.go with http scheme and passing integration tests.

func NewConfiguration() *Configuration {
	cfg := &Configuration{
		BasePath:      "http://petstore.swagger.io:80/v2",
		DefaultHeader: make(map[string]string),
		UserAgent:     "OpenAPI-Generator/1.0.0/go",
	}
	return cfg
}