Fix a bug in parse_document_content.

`peek(stream.input)` is a token instance, so it should be `in(typeof(peek(stream.input)), [A, B, C, D]`.
However, it is better to use `isa` and `Union`, as I mentioned in
JuliaData#161.

src/parser.jl

@@ -214,7 +214,7 @@ end
 
 
 function parse_document_content(stream::EventStream)
-    if in(peek(stream.input), [DirectiveToken, DocumentStartToken, DocumentEndToken,StreamEndToken])
+    if peek(stream.input) isa Union{DirectiveToken, DocumentStartToken, DocumentEndToken,StreamEndToken}
         event = process_empty_scalar(stream, peek(stream.input).span.start_mark)
         stream.state = pop!(stream.states)
         event