Resolve #247: set namespace map directly in SimpleXmlEncoder

Adds a `setNamespaces` setter to `SimpleXmlEncoder` which accepts a map
of strings defining the namespaces for the xml document generated by the
encoder.

Author: cboehme

src/main/java/org/culturegraph/mf/stream/converter/xml/SimpleXmlEncoder.java

@@ -108,7 +108,11 @@ public void setSeparateRoots(final boolean separateRoots) {
 	}
 
 	public void setNamespaces(final MultiMap multimap) {
-		this.namespaces = multimap.getMap(NAMESPACES);
+		setNamespaces(multimap.getMap(NAMESPACES));
+	}
+
+	public void setNamespaces(final Map<String, String> namespaces) {
+		this.namespaces = namespaces;
 	}
 
 	@Override