Do not create a select mode default mapping.

In select mode (the variant of visual mode), any printable characters should override the selection (see below :help v_CTRL-G), so no such mappings should exist. Limit the default visual mode mapping to visual mode (:xmap). (The <Plug>(Exchange) can and should be defined for both.)
Merge pull request #26 from inkarkat/no-select-mode-mapping

Do not create a select mode default mapping.