Fix preview for document created from an XML export

ERROR [PreviewRestlet] Could not build preview for missing blob at default
org.nuxeo.ecm.platform.preview.api.PreviewException: Cannot encode blob content to string
	at org.nuxeo.ecm.platform.preview.adapter.PlainTextPreviewer.getPreview(PlainTextPreviewer.java:68)
	at org.nuxeo.ecm.platform.preview.adapter.base.ConverterBasedHtmlPreviewAdapter.getPreviewBlobs(ConverterBasedHtmlPreviewAdapter.java:128)
	at org.nuxeo.ecm.platform.preview.adapter.base.ConverterBasedHtmlPreviewAdapter.getPreviewBlobs(ConverterBasedHtmlPreviewAdapter.java:113)
	at org.nuxeo.ecm.platform.preview.adapter.base.AbstractHtmlPreviewAdapter.getFilePreviewBlobs(AbstractHtmlPreviewAdapter.java:127)
	at org.nuxeo.ecm.platform.preview.restlet.PreviewRestlet.initCachedBlob(PreviewRestlet.java:204)
	at org.nuxeo.ecm.platform.preview.restlet.PreviewRestlet.handle(PreviewRestlet.java:136)
	...
Caused by: java.io.UnsupportedEncodingException: 
	at java.lang.StringCoding.decode(StringCoding.java:190)
	at java.lang.String.<init>(String.java:452)
	at java.lang.String.<init>(String.java:524)
	at org.nuxeo.ecm.platform.preview.adapter.PlainTextPreviewer.getPreview(PlainTextPreviewer.java:66)
	... 79 more