Ruby

Fix per-instance Regexp timeout (#6621)

Fix per-instance Regexp timeout

This makes it follow what was decided in [Bug #19055]:

• Regexp.new(str, timeout: nil) should respect the global timeout
• Regexp.new(str, timeout: huge_val) should use the maximum value that
  can be represented in the internal representation
• Regexp.new(str, timeout: 0 or negative value) should raise an error