FileTrustStoreSslSocketFactory must support JVM truststore

I discovered a bug with the `FileTrustStoreSslSocketFactory` that I noticed it was improperly loading the custom and default truststores, and failing to combine them. This pull addresses that, and is accompanied by unit tests. 
This is dangerous. The Sun key/trust managers may not be available; for example IBM JVM or custom provider listing in java.security.