Fix Enum#to_s for flag enums to join with pipe

[makenowjust]

See #3961

[bcardiff]

This overrides #3961, right? I don't know why there were commas in the first and i didn't stop in questioning them to avoid the ambiguity the parens solved.

[bcardiff]

I've just read the 3961 thread. Unless something comes up i would also go with pipes :-)

[sdogruyol]

Pipe wins 💯

[makenowjust]

NOTE: Comma separation is corresponding to Enum.flag macro argument. It may be important.

[ysbaddaden]

Good note. I believe Enum.flag to a macro to quickly a flags Enum, I'm not sure it should dictate an Enum representation.

[asterite]

Pipes are good for to_s. I originally copied the comma from to_s from how C# shows these enums, but using pipes is better.