using ::isnan to avoid conflicts with std::isnan for C++11 build.

…uffers#508)

In Go 1.10, go vet and gofmt have changed their output in a way to breaks the current build and test cycle.

gofmt turns a one-line definition of a single-method interface into three lines; this causes a golden
comparison test to produce different results between Go 1.9 and Go 1.10.
go vet is now run by default with go test, which uncovered a bad Stringer argument in extension_test.go.
Tested on Go1.10rc1 and Go1.9.3.

…num value options (no wrapper), fixes protocolbuffers#510; expose zero on LongBits, fixes protocolbuffers#508