Reduce String allocations by storing common seperators in a constant.
Reduce String allocations by storing common seperators in a constant.

Before:
```
[["rack/lib/rack/utils.rb", 87, :T_STRING], [111173, 57, 106670, 0, 27, 12654156]]
[["rack/lib/rack/utils.rb", 115, :T_STRING], [30244, 58, 29580, 0, 27, 3275248]]
```

After:
```
[["rack/lib/rack/utils.rb", 88, :T_STRING], [20979, 136, 21178, 0, 27, 1677320]]
[["rack/lib/rack/utils.rb", 116, :T_STRING], [8995, 137, 9943, 0, 27, 581392]]
```

supzann3/rack,davydovanton/rack,jeremyf/rack,liamseanbrady/rack,gioele/rack,wjordan/rack,trexnix/rack,AMekss/rack,maclover7/rack,tylergannon/rack,liamseanbrady/rack,ganmacs/rack,stepri/rack,supzann3/rack,gfvcastro/rack,eavgerinos/rack,gfvcastro/rack,gioele/rack,keithduncan/rack,eins78/rack,shaneog/rack,nmk/rack,nmk/rack,tylergannon/rack,Comcast/rack,fabianrbz/rack,wjordan/rack,jeremyf/rack,jackxxu/rack,Codeman655/rack,zjsxwc/rack,davidcelis/rack,davydovanton/rack,imtayadeway/rack,imtayadeway/rack,ngpestelos/rack,fabianrbz/rack,wjordan/rack,arronmabrey/rack,keepcosmos/rack,liamseanbrady/rack,keepcosmos/rack,trexnix/rack,keithduncan/rack,akihiro17/rack,maclover7/rack,akihiro17/rack,Comcast/rack,supzann3/rack,eavgerinos/rack,gioele/rack,laplaceliu/rack,davydovanton/rack,ganmacs/rack,shaneog/rack,arronmabrey/rack,AMekss/rack,rwz/rack,gfvcastro/rack,keepcosmos/rack,maclover7/rack,akihiro17/rack,davidcelis/rack,deepj/rack,tylergannon/rack,jackxxu/rack,ganmacs/rack,Codeman655/rack,wildjcrt/rack,wildjcrt/rack,imtayadeway/rack,laplaceliu/rack,davidcelis/rack,zjsxwc/rack,deepj/rack,rwz/rack,Codeman655/rack,shaneog/rack,stepri/rack,rwz/rack,eins78/rack,AMekss/rack,Eric-Guo/rack,fabianrbz/rack,deepj/rack,Eric-Guo/rack,jeremyf/rack,zjsxwc/rack,Comcast/rack,keithduncan/rack,wildjcrt/rack,eavgerinos/rack,arronmabrey/rack,Eric-Guo/rack,stepri/rack,nmk/rack,laplaceliu/rack,ngpestelos/rack,jackxxu/rack,eins78/rack,trexnix/rack,ngpestelos/rack