Running standalone metapost file omits spaces between btex and etex

[Hongwen Qiu <qiuhongwen@gmail.com>]

I found that the following example

\starttext

\startuseMPgraphic{test}
draw btex some text with space etex;
\stopuseMPgraphic
\useMPgraphic{test}

\stoptext

can generate the text between "btex" and "etex" correct no matter I use
MKII or MKIV.

But, when I running the following example

beginfig(0);
draw btex some text with space etex;
endfig;
end

with the command line:

$ mpost --mem=metafun test.mp

or

$ texexec --mptex test.mp

It dose generate output. But the output omits all the spaces between
btex and etex, with the resulting string to be "sometextwithspace".

So, the question is how to get the correct results when running
standalong metapost file with btex-etex strings.

Thanks.
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