move SSE-C TLS enforcement into generic handler (#6639)

This commit moves the check that SSE-C requests
must be made over TLS into a generic HTTP handler.

Since the HTTP server uses custom TCP connection handling
it is not possible to use `http.Request.TLS` to check
for TLS connections. So using `globalIsSSL` is the only
option to detect whether the request is made over TLS.
By extracting this check into a separate handler it's possible
to refactor other parts of the SSE handling code further.

cmd/api-errors_test.go

@@ -52,7 +52,6 @@ var toAPIErrorCodeTests = []struct {
 	{err: errSignatureMismatch, errCode: ErrSignatureDoesNotMatch},
 
 	// SSE-C errors
-	{err: errInsecureSSERequest, errCode: ErrInsecureSSECustomerRequest},
 	{err: crypto.ErrInvalidCustomerAlgorithm, errCode: ErrInvalidSSECustomerAlgorithm},
 	{err: crypto.ErrMissingCustomerKey, errCode: ErrMissingSSECustomerKey},
 	{err: crypto.ErrInvalidCustomerKey, errCode: ErrInvalidSSECustomerKey},