parser: more fixes on stream parsing

* scew/parser.c (parse_stream_buffer_): we use the new parser variable
  parsing_started as a flag to indicate if we have found a non-space
  character that would be the first character to parse in a tree.

* tests/check_parser.c: new chunked stream parsing unit test
  test_load_chunked_stream_b. old one test_load_chunked_stream is now
  test_load_chunked_stream_a.