Fix Description Method Const Correctness

include/rtc/description.hpp

@@ -191,9 +191,9 @@ public:
 		void removeSSRC(uint32_t ssrc);
 		void replaceSSRC(uint32_t old, uint32_t ssrc, optional<string> name,
 		                 optional<string> msid = nullopt, optional<string> trackID = nullopt);
-		bool hasSSRC(uint32_t ssrc);
-		std::vector<uint32_t> getSSRCs();
-		std::optional<std::string> getCNameForSsrc(uint32_t ssrc);
+		bool hasSSRC(uint32_t ssrc) const;
+		std::vector<uint32_t> getSSRCs() const;
+		std::optional<std::string> getCNameForSsrc(uint32_t ssrc) const;
 
 		int bitrate() const;
 		void setBitrate(int bitrate);

src/description.cpp

@@ -776,7 +776,7 @@ void Description::Media::replaceSSRC(uint32_t old, uint32_t ssrc, optional<strin
 	addSSRC(ssrc, std::move(name), std::move(msid), std::move(trackID));
 }
 
-bool Description::Media::hasSSRC(uint32_t ssrc) {
+bool Description::Media::hasSSRC(uint32_t ssrc) const {
 	return std::find(mSsrcs.begin(), mSsrcs.end(), ssrc) != mSsrcs.end();
 }
 
@@ -905,9 +905,9 @@ Description::Media Description::Media::reciprocate() const {
 	return reciprocated;
 }
 
-std::vector<uint32_t> Description::Media::getSSRCs() { return mSsrcs; }
+std::vector<uint32_t> Description::Media::getSSRCs() const { return mSsrcs; }
 
-optional<string> Description::Media::getCNameForSsrc(uint32_t ssrc) {
+optional<string> Description::Media::getCNameForSsrc(uint32_t ssrc) const {
 	auto it = mCNameMap.find(ssrc);
 	if (it != mCNameMap.end()) {
 		return it->second;