rapidjson

Fix warnings on GCC 6 and later (closes #666)

* document.h
  * suppress -Wterminate on GCC 6.x and later
  * simplify warning handling
* schema.h
  * drop RAPIDJSON_NOEXCEPT from GenericSchemaDocument constructor
    (calls RAPIDJSON_NEW anyway)
  * simplify warning handling
    (avoids RAPIDJSON_POP mismatch on Clang)
* encodingtest.cpp, istreamwrappertest.cpp
  * work around -Wdangling-else
* readertest.cpp
  * suppress -Wdangling-else