Fix XML conversion - #1192

Converting sqltype=xml values into Haskell types seem to fail:

*** Exception: Incompatible {errSQLType = "xml", errSQLTableOid = column number 1 is out of range 0..-1
Nothing, errSQLField = "column number 1 is out of range 0..-1
", errHaskellType = "Text", errMessage = "types incompatible"}

This is because Persistent tries to parse it as PersistText (using the Database.PostgreSQL.Simple.FromField instance of Text), however the oid of xml, 142, does not match the oid's of name, text, char, bpchar or varchar, resulting in an Incompatible error.

To fix this, I simply changed the type from Text to (unknown) ByteString.

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