Do not/allow users to opt out canonical header keys in autorest.WithHeaders

[magodo]

Currently, the function autorest.WithHeaders always canonical header keys:

go-autorest/autorest/preparer.go

Lines 137 to 140 in ee71315

	// WithHeaders returns a PrepareDecorator that sets the specified HTTP headers of the http.Request to
	// the passed value. It canonicalizes the passed headers name (via http.CanonicalHeaderKey) before
	// adding them.
	func WithHeaders(headers map[string]interface{}) PrepareDecorator {

Some data plane API (e.g. storage) communicates resource properties via header, the canonical here makes those key case insensitive, while sometimes they do allow sensitive. One example can be found here: hashicorp/terraform-provider-azurerm#20737

[jhendrixMSFT]

go-autorest is almost end-of-life and is only taking critical bugfixes. This functionality has been like this for years, so I don't think we'd change it now.