Fix leftovers in basic_parser corner case:

fix #1734, close #1736

buf_.get() has size `buf_len_`, but only `size` number of octets in that
buffer is valid.

include/boost/beast/http/basic_parser.hpp

@@ -128,6 +128,8 @@ class basic_parser
     template<bool OtherIsRequest>
     friend class basic_parser;
 
+    friend class basic_parser_test;
+
 protected:
     /// Default constructor
     basic_parser() = default;

include/boost/beast/http/impl/basic_parser.hpp

@@ -51,9 +51,9 @@ put(ConstBufferSequence const& buffers,
     }
     // flatten
     net::buffer_copy(net::buffer(
-        buf_.get(), buf_len_), buffers);
+        buf_.get(), size), buffers);
     return put(net::const_buffer{
-        buf_.get(), buf_len_}, ec);
+        buf_.get(), size}, ec);
 }
 
 template<bool isRequest>