Better treatment of SSL short reads:

fix #38

This improves the behavior when encountering a short read:

* Any stream error encountered during a read is converting into
  `http::error::partial_message` if some data was received but
  the message is incomplete.

* Examples squelch SSL short read errors from the logs.

include/boost/beast/http/impl/read.hpp

@@ -43,7 +43,7 @@ parse_until(
     {
         if(parser.got_some())
         {
-            // caller sees EOF on next read
+            // Caller sees EOF on next read
             ec = {};
             parser.put_eof(ec);
             BOOST_ASSERT(ec || parser.is_done());
@@ -55,7 +55,17 @@ parse_until(
         return 0;
     }
     if(ec)
+    {
+        // Upgrade the error if we have a partial message.
+        // This causes SSL short reads (and every other error)
+        // to be converted into something else, allowing the
+        // caller to distinguish an SSL short read which
+        // represents a safe connection closure, versus
+        // a closure with data loss.
+        if(parser.got_some() && ! parser.is_done())
+            ec = error::partial_message;
         return 0;
+    }
     if(parser.is_done())
         return 0;
     if(buffer.size() > 0)

include/boost/beast/http/impl/write.hpp

@@ -105,6 +105,7 @@ public:
             // What else could it be?
             BOOST_ASSERT(sr_.is_done());
         }
+
         return net::post(
             s_.get_executor(),
             beast::bind_front_handler(