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. 
According to http://docs.oracle.com/javase/7/docs/technotes/guides/security/StandardNames.html#KeyManagerFactory, "PKIX" is a standard algorithm name that should be supported by alternative JSSE providers. I recommend switching for better cross-platform support and non-standard security configs (e.g. BC).