added read flag for read/write mapping (#9)

Author: johnpatek

include/mio/detail/mmap.ipp

@@ -226,7 +226,7 @@ inline mmap_context memory_map(const file_handle_type file_handle, const int64_t
     char* mapping_start = static_cast<char*>(::mmap(
             0, // Don't give hint as to where to map.
             length_to_map,
-            mode == access_mode::read ? PROT_READ : PROT_WRITE,
+            mode == access_mode::read ? PROT_READ : PROT_READ | PROT_WRITE,
             MAP_SHARED,
             file_handle,
             aligned_offset));