api: Handle content-length-range policy properly. (#3297)

content-length-range policy in postPolicy API was
not working properly handle it. The reflection
strategy used has changed in recent version of Go.
Any free form interface{} of any integer is treated
as `float64` this caused a bug where content-length-range
parsing failed to provide any value.

Fixes #3295

cmd/api-errors.go

@@ -629,6 +629,8 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 		apiErr = ErrContentSHA256Mismatch
 	case ObjectTooLarge:
 		apiErr = ErrEntityTooLarge
+	case ObjectTooSmall:
+		apiErr = ErrEntityTooSmall
 	default:
 		apiErr = ErrInternalError
 	}