Add tests for filter field

Add pattern tests for enum_virtual_ops and get_account_history for filter filed, which will include:

• following query:

curl -s --data '{"jsonrpc":"2.0", "method":"account_history_api.enum_virtual_ops", "params":{"block_range_begin": 2889020, "block_range_end": 2889021, "operation_begin": 3846274, "limit": 150}, "id":1}' -H 'Content-Type: application/json' http://localhost:8078 

in relation to #19 (closed)

• filter is NULL
• filter is 0
• low_filter is NULL high_filter is NULL
• low_filter is NULL high_filter =0
• low_filter is NULL high_filter != 0
• low_filter = 0 high_filter = 0
• low_filter != 0 high_filter = 0
• low_filter != 0 high_filter is NULL
• low_filter = 0 high_filter is NULL