Make buffers_adapter meet requirements

fix #386

CHANGELOG.md

@@ -9,6 +9,7 @@ Version 44
 * Fix README websocket example
 * Add buffers_adapter regression test
 * Tidy up is_dynamic_buffer traits test
+* Make buffers_adapter meet requirements
 
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include/beast/core/buffers_adapter.hpp

@@ -113,6 +113,13 @@ public:
         return in_size_;
     }
     
+    /// Returns the maximum sum of the sizes of the input sequence and output sequence the buffer can hold without requiring reallocation.
+    std::size_t
+    capacity() const
+    {
+        return max_size_;
+    }
+
     /** Get a list of buffers that represents the output sequence, with the given size.
 
         @throws std::length_error if the size would exceed the limit