Added unit test for Guid element

Author: magneticflux-

src/test/groovy/com/github/magneticflux/rss/namespaces/standard/elements/GuidTest.groovy

@@ -0,0 +1,23 @@
+package com.github.magneticflux.rss.namespaces.standard.elements
+
+import com.github.magneticflux.rss.PersisterSpecification
+import spock.lang.Unroll
+
+class GuidTest extends PersisterSpecification {
+	@Unroll
+	def 'enclosure #object read/write'() {
+
+		expect: 'the output XML equals the given XML'
+		persister.write(object) == xml
+
+		and: 'the input object equals the given object'
+		persister.read(Guid, xml) == object
+
+		where:
+		isPermaLinkRaw | text       | xml
+		'true'         | 'testText' | """<guid isPermaLink="$isPermaLinkRaw">$text</guid>"""
+		'false'        | 'testText' | """<guid isPermaLink="$isPermaLinkRaw">$text</guid>"""
+		null           | 'testText' | """<guid>$text</guid>"""
+		object = new Guid(isPermaLinkRaw, text)
+	}
+}