Add non-const string buffer() overloads.

asio/include/asio/buffer.hpp

diff --git a/asio/include/asio/buffer.hpp b/asio/include/asio/buffer.hpp
index b275869..2cc06bb 100644
--- a/asio/include/asio/buffer.hpp
+++ b/asio/include/asio/buffer.hpp
@@ -1226,6 +1226,55 @@
         ));
 }
 
+/// Create a new modifiable buffer that represents the given string.
+/**
+ * @returns <tt>mutable_buffers_1(data.size() ? &data[0] : 0,
+ * data.size() * sizeof(Elem))</tt>.
+ *
+ * @note The buffer is invalidated by any non-const operation called on the
+ * given string object.
+ */
+template <typename Elem, typename Traits, typename Allocator>
+inline mutable_buffers_1 buffer(
+    std::basic_string<Elem, Traits, Allocator>& data)
+{
+  return mutable_buffers_1(mutable_buffer(data.size() ? &data[0] : 0,
+        data.size() * sizeof(Elem)
+#if defined(ASIO_ENABLE_BUFFER_DEBUGGING)
+        , detail::buffer_debug_check<
+            typename std::basic_string<Elem, Traits, Allocator>::iterator
+          >(data.begin())
+#endif // ASIO_ENABLE_BUFFER_DEBUGGING
+        ));
+}
+
+/// Create a new non-modifiable buffer that represents the given string.
+/**
+ * @returns A mutable_buffers_1 value equivalent to:
+ * @code mutable_buffers_1(
+ *     data.size() ? &data[0] : 0,
+ *     min(data.size() * sizeof(Elem), max_size_in_bytes)); @endcode
+ *
+ * @note The buffer is invalidated by any non-const operation called on the
+ * given string object.
+ */
+template <typename Elem, typename Traits, typename Allocator>
+inline mutable_buffers_1 buffer(
+    std::basic_string<Elem, Traits, Allocator>& data,
+    std::size_t max_size_in_bytes)
+{
+  return mutable_buffers_1(
+      mutable_buffer(data.size() ? &data[0] : 0,
+        data.size() * sizeof(Elem) < max_size_in_bytes
+        ? data.size() * sizeof(Elem) : max_size_in_bytes
+#if defined(ASIO_ENABLE_BUFFER_DEBUGGING)
+        , detail::buffer_debug_check<
+            typename std::basic_string<Elem, Traits, Allocator>::iterator
+          >(data.begin())
+#endif // ASIO_ENABLE_BUFFER_DEBUGGING
+        ));
+}
+
 /// Create a new non-modifiable buffer that represents the given string.
 /**
  * @returns <tt>const_buffers_1(data.data(), data.size() * sizeof(Elem))</tt>.

asio/src/tests/unit/buffer.cpp

diff --git a/asio/src/tests/unit/buffer.cpp b/asio/src/tests/unit/buffer.cpp
index 8c64524..0ba4317 100644
--- a/asio/src/tests/unit/buffer.cpp
+++ b/asio/src/tests/unit/buffer.cpp
@@ -57,7 +57,8 @@
 #endif // defined(ASIO_HAS_STD_ARRAY)
     std::vector<char> vector_data(1024);
     const std::vector<char>& const_vector_data = vector_data;
-    const std::string string_data(1024, ' ');
+    std::string string_data(1024, ' ');
+    const std::string const_string_data(1024, ' ');
     std::vector<mutable_buffer> mutable_buffer_sequence;
     std::vector<const_buffer> const_buffer_sequence;
 
@@ -162,8 +163,10 @@
     mb1 = buffer(vector_data, 1024);
     cb1 = buffer(const_vector_data);
     cb1 = buffer(const_vector_data, 1024);
-    cb1 = buffer(string_data);
-    cb1 = buffer(string_data, 1024);
+    mb1 = buffer(string_data);
+    mb1 = buffer(string_data, 1024);
+    cb1 = buffer(const_string_data);
+    cb1 = buffer(const_string_data, 1024);
 
     // buffer_copy function overloads.