Parsing XML from IOBuffer

[alhirzel]

Wondering if readxml(::IOBuffer) is a valid use case?

# works
xml = "<text>Hello, World</text>"
@show parsexml(xml)

# ERROR: LoadError: XMLError: Document is empty from XML parser (code: 4, line: 1)
io = IOBuffer()
write(io, xml)
@show readxml(io)

I think the current readxml function may be dependent on the layout of IOStream? Perhaps the following needs to be adapted to work for ::IO rather than just ::IOStream:

EzXML.jl/src/document.jl

Lines 147 to 160 in 8cc2855

	function readxml(input::IO)
	readcb = make_read_callback()
	closecb = C_NULL
	context = pointer_from_objref(input)
	uri = C_NULL
	encoding = C_NULL
	options = 0
	doc_ptr = @check ccall(
	(:xmlReadIO, libxml2),
	Ptr{_Node},
	(Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}, Cstring, Cstring, Cint),
	readcb, closecb, context, uri, encoding, options) != C_NULL
	return Document(doc_ptr)
	end

I think the context argument needs to be a C FILE *? It was difficult for me to find clear documentation for libxml2. I'm happy to PR a fix+tests once I understand what to do with an IOBuffer.