[render] Content-Type charset

As a best practise, I think we should add "charset=UTF-8" to the JSON and HTML content-types in the package. Two ways that this could be implemented. The first and by far the simplest is to just updated the constants like so:

```go
    ContentJSON    = "application/json; charset=UTF-8"
    ContentHTML    = "text/html; charset=UTF-8"
```

If anyone has objections to this, I would then suggest we add a new Options param called "AppendCharset" which would be a bool (defaulted to false). This would allow the developer to set it to true, and have the "; charset=UTF-8" automatically appended when we set the header.

Thoughts?