Fix -Weffc++ in the BSON recorder test and a clang-tidy use-auto hit

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/binary_reader.hpp

@@ -382,7 +382,7 @@ class binary_reader
 
         // the declared size bounds where the document ends: an embedded
         // document must end before the terminator of the document that contains it
-        const std::size_t unsigned_size = static_cast<std::size_t>(document_size);
+        const auto unsigned_size = static_cast<std::size_t>(document_size);
         const std::size_t document_end = unsigned_size > (std::numeric_limits<std::size_t>::max)() - document_start
                                          ? (std::numeric_limits<std::size_t>::max)()
                                          : document_start + unsigned_size;