explicity close url connection once request is completed

Occasionally, I get an error like `javax.net.ssl.SSLException: Write error: ssl=0x594e9b00: I/O error during system call, Broken pipe`.

Apparently, this happens when Android attempts to recycle old connections (SSL connections are quite expensive to create, but the overhead to keep them around isn't so bad). I fixed this my explicitly closing the connection after the request is completed.