It parse Content-Type header and return a copy of the data
Return the whole string of the header body for now
Application just need to see if the returned Data has
substring of "application/sdp" to know if it support sdp
TODO: try parse the detail of Content-Type and Accept
Following get functions are getting the info for *first* SdpMedia
If you want to get more than one SdpMedeia,
you need to use getSdpSession to get the list of SdpMedia
get the contentData (msgBody) from a RtspMsg
RtspSdp constructor will decode it and check it if valid
RtspSdp provides get functons for application to read the sdp info
This can be used to decode the SDP in ANNOUNCE
Following set functions are setting data for the *first* SdpMedia
it creates a SdpMedia if there is none
If you want to add more than one SdpMedeia,
you need to create SdpMedia by yourself and use getSdpSession
to add onto it