RTP header length inconsistent with RFC 3550

[chumvan]

As specified in RFC3550 - section 5.1 RTP Fixed Header Field:

The first twelve octets are present in every RTP packet, while the
list of CSRC identifiers is present only when inserted by a mixer.

the header length of an RTP packet should be at least 12 octets hence should include the first 3 rows as can be seen from the figure below (from V to the SSRC):

Currently in pion/rtp/packet.go, we have headerLength = 4

Is there any specific reason for this value ? Otherwise, it should be adjusted to maintain the coherence with the standardization.