Use StringBuilder instead of concatenation

Pointed out by ctrueden: string concatenation has been showed to have
significant overhead in time and space, thus we should be using
StringBuilder.

This also removes the potential for a null header value.

Author: hinerm

src/main/java/org/scijava/script/DefaultScriptHeaderService.java

@@ -50,17 +50,15 @@ public class DefaultScriptHeaderService extends
 
 	@Override
 	public String getHeader(final ScriptLanguage language) {
-		String header = null;
+		StringBuilder header = new StringBuilder();
 		for (final ScriptHeader scriptHeader : getInstances()) {
 			if (scriptHeader.supports(language)) {
-				if (header == null) {
-					header = "";
-				}
-				header += scriptHeader.getHeader() + "\n";
+				header.append(scriptHeader.getHeader());
+				header.append("\n");
 			}
 		}
 
-		return header;
+		return header.toString();
 	}
 
 	// -- HandlerService methods --

src/main/java/org/scijava/script/ScriptHeaderService.java

@@ -55,8 +55,7 @@ public interface ScriptHeaderService extends
 	 * {@link ScriptHeader#getHeader()} output to a single string.
 	 *
 	 * @param language - Language to look up
-	 * @return The combined header text to insert at the top of a script, or null
-	 *         if no {@link ScriptHeader}s were found.
+	 * @return The combined header text to insert at the top of a script.
 	 */
 	String getHeader(final ScriptLanguage language);
 }