Sanitize BaseUrl on write.

[paulomorgado]

Currently, BaseUrl is sanitized every time it is used:

urlBuilder_.Append({% if UseBaseUrl %}BaseUrl != null ? BaseUrl.TrimEnd('/') : "").Append("/{% else %}"{% endif %}{{ operation.Path }}{% if operation.HasQueryParameters %}?{% endif %}");

By sanitizing its value when set:

{% if UseBaseUrl and GenerateBaseUrlProperty -%}
    public string BaseUrl
    {
        get { return _baseUrl; }
        set
        {
            _baseUrl = value;
            if (!string.IsNullOrEmpty(_baseUrl) && !_baseUrl.EndsWith("/"))
                baseUrl += '/';
        }
    }

{% endif -%}

it can be used without changes:

        {% if UseBaseUrl %}if (!string.IsNullOrEmpty(BaseUrl)) urlBuilder_.Append(BaseUrl);{% endif %}
        urlBuilder_.Append("{{ operation.Path }}{% if operation.HasQueryParameters %}?{% endif %}");

This is a runtime performance improvement for those using base URL.

[paulomorgado]

Sorry about the bugs on this PR, everyone.

@MaximeMorin-Devolutions, I've created #4570. Can you check the changes and propose changes for anything else you found?