Xpp3DomUtils#mergeIntoXpp3Dom() must not override the dominant value in case it is blank

[kwin]

Despite the javadoc saying that

if the dominant root node's value is empty, set it to the recessive root node's value

which is clearly not true for blank values (i.e. length() > 0), still the dominant root node's value is overwritten by the recessive one in case it is blank(!) due to the trim logic in

plexus-utils/src/main/java/org/codehaus/plexus/util/xml/Xpp3DomUtils.java

Line 285 in 3896620

return ( str == null || str.trim().length() == 0 );

.

As the Xpp3DomBuilder trims by default it seems like a bug to me that merging two Xpp3Dom s (usually created by Xpp3DomBuilder) should trim again before merging as this would only make a difference in case someone set xml:space="preserve" in the dominant Xpp3Dom which should IMHO take precedence.

[kwin]

I added a failing test case and fix in #213.