Fix missing pragma GCC diagnostic push

gcc wasn't complaining but clang is.
Enable tests fixed by #21

These test cases were disabled in 470c8a235928d1100
but fixed as a side effect in 6888ceb (#21).
Merge pull request #30 from cesanta/enable_tests

Enable tests fixed by #21
Merge pull request #29 from cesanta/gcc_pragma

Fixe missing pragma GCC diagnostic push
Fix 'new' operator for String, fixes #6

It was setting the string proto to the global scope proto
instead of creating a new object.
Merge pull request #31 from cesanta/new_op

Fix 'new' operator for String