Header parameters are not being handled as case-insensitive

[brocoli]

Headers field names are case-insensitive as per RFC7230, but they're being treated as case-sensitive in requests.

The root of the problem seems to be that in oas3/parameterParsers/index.ts, many parameters go through the toStructuredParser function, that does const value = rawParamValues[location.name]. rawParamValues has the header keys already in lower-case, while location.name is still in Train-Case.

I've got a PR to fix this in the works, though I'm not sure if the way I solved this was the best approach possible. I copied the one used for query parameters -- to call a different function in oas3/Operation.ts in parseParameters for them and, do a little preprocessing -- but it feels a little redundant to do this for each location (i.e. in: query / in: header), when we already have custom parsers that vary by style / format.

[jwalton]

Your fix here certainly looks reasonable to me. Open a PR and I'll merge it. :)