Added hash_value for buffer.

Author: redboltz

include/mqtt/buffer.hpp

@@ -83,6 +83,12 @@ class buffer : public MQTT_NS::string_view {
     const_shared_ptr_array lifetime_;
 };
 
+inline std::size_t hash_value(buffer const& v) {
+    std::size_t ret = 0;
+    boost::hash_combine(ret, static_cast<MQTT_NS::string_view const&>(v));
+    return ret;
+}
+
 inline namespace literals {
 
 /**