Refactor buffers_range:

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

include/boost/beast/core/detail/buffers_range.hpp

@@ -46,9 +46,13 @@ public:
             buffer_sequence_iterator<BufferSequence>::type;
 
         iter_type it_;
+        buffers_range_adaptor const* b_ = nullptr;
 
-        const_iterator(iter_type const& it)
+        const_iterator(
+            buffers_range_adaptor const& b,
+            iter_type const& it)
             : it_(it)
+            , b_(&b)
         {
         }
 
@@ -60,17 +64,19 @@ public:
         using difference_type = std::ptrdiff_t;
         using iterator_category =
             std::bidirectional_iterator_tag;
-        
+
+        const_iterator() = default;
+
         bool
         operator==(const_iterator const& other) const
         {
-            return it_ == other.it_;
+            return b_ == other.b_ && it_ == other.it_;
         }
 
         bool
         operator!=(const_iterator const& other) const
         {
-            return ! (*this == other);
+            return !(*this == other);
         }
 
         reference
@@ -97,7 +103,6 @@ public:
             return temp;
         }
 
-        // deprecated
         const_iterator&
         operator--()
         {
@@ -105,7 +110,6 @@ public:
             return *this;
         }
 
-        // deprecated
         const_iterator
         operator--(int)
         {
@@ -121,16 +125,21 @@ public:
     {
     }
 
+    buffers_range_adaptor(
+        buffers_range_adaptor const&) = default;
+    buffers_range_adaptor& operator=(
+        buffers_range_adaptor const&) = default;
+
     const_iterator
     begin() const noexcept
     {
-        return net::buffer_sequence_begin(b_);
+        return {*this, net::buffer_sequence_begin(b_)};
     }
 
     const_iterator
     end() const noexcept
     {
-        return net::buffer_sequence_end(b_);
+        return {*this, net::buffer_sequence_end(b_)};
     }
 };