Add specs for rb_scan_args that shouldn't emit warnings when passed nil as last argument

```
require 'date'

DateTime.rfc2822(nil)

# The last argument is nil, treating as empty keywords
# .../lib/truffle/truffle/cext.rb:1132:in `rb_exc_raise': invalid date (Date::Error)
```

Author: andrykonchin

spec/ruby/library/datetime/rfc2822_spec.rb

@@ -3,4 +3,8 @@
 
 describe "DateTime.rfc2822" do
   it "needs to be reviewed for spec completeness"
+
+  it "raises DateError if passed nil" do
+    -> { DateTime.rfc2822(nil) }.should raise_error(Date::Error, "invalid date")
+  end
 end

spec/ruby/optional/capi/util_spec.rb

@@ -127,6 +127,19 @@
       ScratchPad.recorded.should == [1, 7, 4]
     end
 
+    it "assigns optional arguments with no hash argument given" do
+      @o.rb_scan_args([1, 7], "02:", 3, @acc).should == 2
+      ScratchPad.recorded.should == [1, 7, nil]
+    end
+
+    it "assigns optional arguments with no hash argument given and rejects the use of optional nil argument as a hash" do
+      -> {
+        @o.rb_scan_args([1, nil], "02:", 3, @acc).should == 2
+      }.should_not complain
+
+      ScratchPad.recorded.should == [1, nil, nil]
+    end
+
     it "assigns required, optional, splat, post-splat, Hash and block arguments" do
       h = {a: 1, b: 2}
       @o.rb_scan_args([1, 2, 3, 4, 5, h], "k11*1:&", 6, @acc, &@prc).should == 5

spec/tags/optional/capi/util_tags.txt

@@ -2,3 +2,4 @@ fails:C-API Util function rb_scan_args rejects the use of nil as a hash
 fails:C-API Util function rb_scan_args does not reject non-symbol keys in keyword arguments
 fails:C-API Util function rb_scan_args does not reject non-symbol keys in keyword arguments with required argument
 fails:C-API Util function rb_scan_args considers keyword arguments with non-symbol keys as keywords when using splat and post arguments
+fails:C-API Util function rb_scan_args assigns optional arguments with no hash argument given and rejects the use of optional nil argument as a hash