[BUG] Space is consumed if it is a first symbol of second %{} group that contains other characters as well

[ZyX]

The following code will show `||' in a statusline while it is expected to show 
`| |':
    vim -u NONE -c 'set laststatus=2' -c 'let &statusline="%{\"|\"}%{\" |\"}"'
None of the following code have this issue:
    vim -u NONE -c 'set laststatus=2' -c 'let &statusline="%{\"|\"}%{\" \"}|"'
    vim -u NONE -c 'set laststatus=2' -c 'let &statusline="|%{\" |\"}"'
    vim -u NONE -c 'set laststatus=2' -c 'let &statusline="|%{\" \"}|"'
tested on vim-7.3.47 (revision df6b12c84b23).

Bug found by Sarah (http://stackoverflow.com/questions/4067613).

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