Marshal.load: restore instance variables on Regexp

	[Bug #19439]

	The instance variables were restore on the Regexp source,
	not the regexp itself.

	Unfortunately we have a bit of a chicken and egg problem.

	The source holds the encoding, and the encoding need to be set on
	the source to be able to instantiate the Regexp.

	So the instance variables have to be read on the `source`.
	To correct this we transfert the instance variables after
	instantiating the Regexp.

	The only way to avoid this would be to read the instance variable
	twice and rewind.
	---
	 marshal.c                             | 20 ++++++++++++++++++--
	 spec/ruby/core/marshal/shared/load.rb | 11 +++++++++++
	 2 files changed, 29 insertions(+), 2 deletions(-)