Fix MemoryStream::Peek() and add test for fix

MemoryStream::Peek() did not return '\0' if src_ == end_, but Peek() == '\0' is
used in parsing in the GenericReader. Without this change, parsing with
MemoryStream as the InputStream could result in a segmentation fault.
Merge pull request #166 from spl/fix-memorystream-peek

Fix MemoryStream::Peek() and add test for fix