Make String#{strip,lstrip}{,!} strip leading NUL bytes

	The documentation already specifies that they strip whitespace
	and defines whitespace to include null.

	This wraps the new behavior in the appropriate guards in the specs,
	but does not specify behavior for previous versions, because this
	is a bug that could be backported.

	Fixes [Bug #17467]
	---
	 spec/ruby/core/string/lstrip_spec.rb | 18 ++++++++++++------
	 spec/ruby/core/string/strip_spec.rb  | 22 ++++++++++------------
	 string.c                             |  4 ++--
	 test/ruby/test_string.rb             | 16 ++++++++++++++++
	 4 files changed, 40 insertions(+), 20 deletions(-)