fix(94): fix formatting to be compliant with XMLBuilder

Please include a summary of the change and which issue is fixed. Fixes # (issue number)

Type of change:

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation update

This PR should fix previously reported issue https://github.com/oozcitak/xmlbuilder2/issues/94 that i also encountered. current pretty print in XMLBuilderCB prints values on separate lines like:

<Root>
   <Test>
   value
   </Test>
</Root>

but it should be

<Root>
   <Test>value</Test>
</Root>

the main problem is that pretty printed xml in that way may not pass xsd validation, because printer adds spaces and '\n's.

This might be a naive approach, so i would appreciate review from codeowner

oozcitak / xmlbuilder2