Fix xmlCtxtReadDoc with encoding

xmlCtxtReadDoc used to create an input stream involving
xmlNewStringInputStream. This would create a stream without an input
buffer, causing problems with encodings (see #34).

After commit aab584dc3, an error was returned even with UTF-8 encodings
which happened to work before.

Make xmlCtxtReadDoc call xmlCtxtReadMemory which doesn't suffer from
these issues. Also fix htmlCtxtReadDoc.

Fixes #397.