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;