Refactor buffers_range:

* Add tests
* Increase code coverage
* Correct behavior for default constructed iterators

test/beast/core/buffers_range.cpp

@@ -9,3 +9,46 @@
 
 // Test that header file is self-contained.
 #include <boost/beast/core/buffers_range.hpp>
+
+#include "buffer_test.hpp"
+
+#include <boost/beast/_experimental/unit_test/suite.hpp>
+
+namespace boost {
+namespace beast {
+
+class buffers_range_test : public beast::unit_test::suite
+{
+public:
+    BOOST_STATIC_ASSERT(
+        net::is_mutable_buffer_sequence<
+            decltype(beast::buffers_range(
+                std::declval<net::mutable_buffer>()))>::value);
+
+    void
+    testBufferSequence()
+    {
+        {
+            string_view s = "Hello, world!";
+            test_buffer_sequence(*this, buffers_range(
+                net::const_buffer{s.data(), s.size()}));
+        }
+        {
+            char buf[13];
+            test_buffer_sequence(*this,
+                buffers_range(net::mutable_buffer{
+                    buf, sizeof(buf)}));
+        }
+    }
+
+    void
+    run() override
+    {
+        testBufferSequence();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(beast,core,buffers_range);
+
+} // beast
+} // boost