bugfix: some whitespaces disappear around substitution expressions

[erdos]

In some cases whitespace characters disappear around substitutions in Microsoft Office Word.

For example consider the following dummy xml content:

<a>Amount: {%=x</a><a>%} pieces</a>

After substitution it gives:

<a>Amount: 23</a><a> pieces</a>

The second <a> starts with a whitespace character, however, Word does not display that.

This bugfix works the following way:

• In a postprocess steps it looks for t elements with text content starting or ending with a whitespace character and adds an xml:whitespace=preserve attribute to them.