Fix defaultMergeStrategy for META-INF/*.RSA signature files

I've found RSA signatures in the following artifacts:
```
org.eclipse.jetty.orbit/javax.transaction/orbits/javax.transaction-1.1.1.v201105210645.jar:META-INF/ECLIPSEF.RSA
org.eclipse.jetty.orbit/javax.servlet/orbits/javax.servlet-3.0.0.v201112011016.jar:META-INF/ECLIPSEF.RSA
org.eclipse.jetty.orbit/javax.mail.glassfish/orbits/javax.mail.glassfish-1.4.1.v201005082020.jar:META-INF/ECLIPSEF.RSA
org.eclipse.jetty.orbit/javax.activation/orbits/javax.activation-1.1.0.v201105071233.jar:META-INF/ECLIPSEF.RSA
```
It is good to discard them when merge like other signatures.