Refactor StringData::capacity() to not count the null terminator

Summary: This makes it consistent with std::string() and the general design
that String is an array of bytes, not a null-terminated c-string.

Reviewed By: @jdelong

Differential Revision: D1681702

Signature: t1:1681702:1416272460:8bf88db48f411c1868f23901bb1ec2c5ea1cd937