Allow RegExp to String coercion

Closes tc39#3

spec.emu

@@ -12,7 +12,9 @@ contributors: Jordan Harband
 			1. Let _matcher_ be ? GetMethod(_regexp_, @@matchAll).
 			1. If _matcher_ is not *undefined*, then
 				1. Return ? Call(_matcher_, _regexp_, « _O_ »).
-		1. Throw a *TypeError* exception.
+		1. Let _S_ be ? ToString(_O_).
+		1. Let _rx_ be ? RegExpCreate(_regexp_, *undefined*).
+		1. Return ? Invoke(_rx_, @@matchAll, « _S_ »).
 	</emu-alg>
 	<emu-note>Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.</emu-note>
 </emu-clause>