Additionally: The return value of `getHeaders()` can only be an array if duplicate header-lines with the same field-name are represented as the field-value in that "field-name : field-value" pair in the same way as RFC 2616 standardizes these.

This is also fuzzy in the `HeaderValuesInterface` that acts as if there could be multiple field-values for the same field-name which is not correct. This is only the case in the message-header-fields, not for concrete field-values.