[MS.Build] now ITaskItem impl. in BuildEngine4 behaves consistent with MS.Build.Tasks.

To be precise, now the implementation returns ItemSpec for ToString().
WriteLinesIntoFile task writes ITaskItem[] lines into a file, without calling
ItemSpec.
Maybe it is better to fix Tasks.dll but I'm unsure about xbuild compatibility.

This missing ToString() override caused target framework monikor output
to become invalid source file (i.e. it used to write ItemSpec as code(!) but
this ITaskItem impl. in BuildEngine4 just wrote its type name as .ToString()
result).