newline in mtext produces "unknown character" warning [was: Newlines not properly handled inside `\text`, `\textrm` and similar macros]

Created by: jonasmalacofilho

Depending inside where a newline appears, it may cause "unknown char" errors.

For instance, while

$$\textrm{foo 
bar}$$

generates "SVG Jax - unknown char" messages, a similar

$${foo 
bar)$$

works without warnings.

---

MathJax: v2.7.0
Client: Chrome 55/Node.JS v7.4.0