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. 
Not any that I found. Combining multiple truststores isn't apparently that common. The solution you see in this PR is what everyone I found suggested to use, but not in form a particular library. Would be great if there was one.