data in-place. When len is not
null it will contain the length of the decoded data.
data
.
header to a RTSPHeaderField.
header field is unknown.
method to a RTSPMethod. method is
unknown.
field may appear multiple times in a message.
field to a string.
method to a string.
options to a string.
code to a string.
result in a human readable string.
version to a string.
