Set UTF-8 encoding to '@include' result string

Fluentd assumes configuration file encoding is UTF-8.
@include is parsed after set Encoding.default_xxternal = 'ASCII-8BIT' so
@include result becomes ASCII-8BIT encoding. It causes encoding mismatch.
Merge pull request #355 from fluent/fix-v1-include-encoding

Set UTF-8 encoding to '@include' result string