Return std::size_t from Body::writer::put (API Change)

`put` returns the number of bytes actually transferred
from the input buffers.

Actions Required:

* Return the number of bytes actually transferred from the
  input buffers in user defined `Body::writer::put` functions.

example/common/file_body.hpp

@@ -255,7 +255,7 @@ public:
     // buffer sequences corresponding to the incoming body.
     //
     template<class ConstBufferSequence>
-    void
+    std::size_t
     put(ConstBufferSequence const& buffers, beast::error_code& ec);
 
     // This function is called when writing is complete.
@@ -307,16 +307,20 @@ writer(beast::http::message<isRequest, file_body, Fields>& m,
 // This will get called one or more times with body buffers
 //
 template<class ConstBufferSequence>
-void
+std::size_t
 file_body::writer::
 put(ConstBufferSequence const& buffers, beast::error_code& ec)
 {
+    // This function must return the total number of
+    // bytes transferred from the input buffers.
+    std::size_t bytes_transferred = 0;
+
     // Loop over all the buffers in the sequence,
     // and write each one to the file.
     for(boost::asio::const_buffer buffer : buffers)
     {
         // Write this buffer to the file
-        fwrite(
+        bytes_transferred += fwrite(
             boost::asio::buffer_cast<void const*>(buffer), 1,
             boost::asio::buffer_size(buffer),
             file_);
@@ -327,12 +331,14 @@ put(ConstBufferSequence const& buffers, beast::error_code& ec)
             // Convert the old-school `errno` into
             // an error code using the generic category.
             ec = beast::error_code{errno, beast::generic_category()};
-            return;
+            return bytes_transferred;
         }
     }
 
     // Indicate success
     ec = {};
+
+    return bytes_transferred;
 }
 
 // Called after writing is done when there's no error.

example/common/mutable_body.hpp

@@ -120,7 +120,7 @@ struct mutable_body
         }
 
         template<class ConstBufferSequence>
-        void
+        std::size_t
         put(ConstBufferSequence const& buffers,
                 beast::error_code& ec)
         {
@@ -135,10 +135,10 @@ struct mutable_body
             catch(std::length_error const&)
             {
                 ec = beast::http::error::buffer_overflow;
-                return;
+                return 0;
             }
             ec.assign(0, ec.category());
-            buffer_copy(boost::asio::buffer(
+            return buffer_copy(boost::asio::buffer(
                 &body_[0] + len, n), buffers);
         }