Fix re-embedding of strings during compaction

	The reference updating code for strings is not re-embedding strings
	because the code is incorrectly wrapped inside of a
	`if (STR_SHARED_P(obj))` clause. Shared strings can't be re-embedded
	so this ends up being a no-op. This means that strings can be moved to a
	large size pool during compaction, but won't be re-embedded, which would
	waste the space.
	---
	 gc.c                         | 16 +++++++++-------
	 string.c                     | 12 ++++++++----
	 test/ruby/test_gc_compact.rb |  8 ++++----
	 3 files changed, 21 insertions(+), 15 deletions(-)