Include the header in ExtendedReport.MarshalSize

[RaphaelFakhri]

Include the header in ExtendedReport.MarshalSize

Description

ExtendedReport.MarshalSize returned wireSize(x), which excludes the 4-byte
RTCP header that Marshal writes. The result was 4 bytes smaller than the
marshaled packet, and CompoundPacket.MarshalSize was short by the same amount
for each extended report.

MarshalSize now adds headerLength, which matches the other packet types.

Tests

• Added TestExtendedReportMarshalSize, which compares MarshalSize with the
  marshaled length for an ExtendedReport and for a CompoundPacket that
  contains one. It fails without the change (expected 208, actual 204) and
  passes with it.
• go test -race ./... and golangci-lint run ./... pass.