SSML generator should escape XML control characters

[Robertof]

Hey,

first of all - thanks for your library! I encountered an issue during the use of SSMLGenerator. By doing something like:

$generator = new SsmlGenerator();
$generator->say ("This & That");
print $generator->getSsml(), "\n";

An invalid SSML document is generated (and refused by Alexa), as it contains the "&" XML control character unescaped.
This reference should be useful. I think that using htmlspecialchars on the input provided to say() should be sufficient, I'll investigate.

Thanks,
Roberto

[maxbeckers]

Good point. Could be helpful in some cases. Wanna fix it and create a PR?

[Robertof]

Alright, I'll take care of it in the coming days!

[maxbeckers]

Fine, Thanks 👍