Fix Writers returning n-1 bytes written.

ByteWriter and StringWriter were returning n-1 bytes written which was
causing io.Copy to error out with a ShortWrite. This would cause files
that were larger than the io.Copy internal buffer (32 * 1024) to stop
after the first chunk.

bytewriter.go

@@ -31,5 +31,7 @@ func (w *ByteWriter) Write(p []byte) (n int, err error) {
 		w.c++
 	}
 
+	n++
+
 	return
 }

stringwriter.go

@@ -31,5 +31,7 @@ func (w *StringWriter) Write(p []byte) (n int, err error) {
 		w.c++
 	}
 
+	n++
+
 	return
 }