(TK-45) Add string support to `:scheme:` option

Allow the `:scheme` option for a proxy to take a string
as well as a keyword.
OK.  Well, I still think that's probably an edge use case.  They must have had to create their own config option for that because the j9 service doesn't have a config option for that.  I think it's probably more common that these will be configured via code, so I vote for continuing to support the keywords.