replace use of '<<' operator with '+=' operator in find_child_section…
…s and find_decendant_sections in SiteDrop
We found that mutiple calls to the 'child_sections' drop filter was modifying the value of the path
attribute inside of SectionDrops. The SiteDrop methods find_child_sections and find_decentdent_sections
used the '<<' operator to append a '/' charecter to the path. This had the side effect of modifying
the original string in memory. The "+=" while slower, uses a copy of the string passed as parameter.
The gave us the behavior we would expect.