Incorrect result of nested mtl:for

[Maxim Reznik]

Substitution of nested mtl:for is incorrect.

See test for #454 where 'expected' text is wrong:

     post 1
       post 2 -> subitem 1-2
       post 2 -> subitem 1-2
        post 2
       post 2 -> subitem 2-2
       post 2 -> subitem 2-2

Correct value should be:

     post 1
       post 1 -> subitem 1-1
       post 1 -> subitem 1-2
        post 2
       post 2 -> subitem 2-1
       post 2 -> subitem 2-2

[Maxim Reznik]

In 5859:

Flush all accamulated characters at the end of loop

To apply any substitutions in them. Closes #496

[vadim.godunko]

Milestone renamed