public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType) {
        return mediaType.equals(MediaType.TEXT_PLAIN_TYPE) && Metadata.class.isAssignableFrom(type);
    }
Returns simple text string for a particular metadata value.
This assumes that the metadata object only has one key;
if there is more than one key or no keys, this will throw a webapp exception.
<p/>
This will choose the first value returned for the one key.
tika-server/tika-server-core/src/main/java/org/apache/tika/server/core/writer/TextMessageBodyWriter.java