Add support for `preserveHeaderCase`

[natebosch]

On the SDK we now have an option to pass preserveHeaderCase when setting Http headers to work around the default behavior of converting them all to lowercase. Some (non spec compliant?) servers expect headers with specific capitalization.

I don't know if there is any equivalent behavior or concern on the web.

Some design options:

• An argument when constructing an IOClient to make the decision once at a client level.
  • Pro: Non breaking.
  • Con: Could be clumsy to use - cross platform code can't reference IOClient, may need to construct a specific client for some requests and a different client for others.
• Add a named argument to all APIs.
  • Pro: fine-grained control by request.
  • Con: If the behavior can't be supported on the web we'd have to ignore the argument.
• Preserve header case on the VM by default with no option to switch.
  • Con: Potentially breaking behavior change for existing users.

[natebosch]

We could also consider flipping the default in the next major version since it seems that most folks who care would prefer to preserve case.

[PiotrWpl]

Hi, any progress with this one?

[natebosch]

@brianquinlan - WDYT about using preserveHeaderCase by default, and/or adding an option? I don't think this is something we can control on the web - is it possible on the other clients?

[brianquinlan]

Setting preserveHeaderCase: true for IOClient seems reasonable to me. But this isn't something that we should require of clients because:

1. case insensitivity is required by RFC-2616
2. some clients (e.g. package:cupertino_http) cannot promise to to preserve case because their underlying implementations are not case preserving