add key check for ParameterCollection
add key check for ParameterCollection


Former-commit-id: 9bd618236b00c256107d65a55c5148e9eac3311f
danielhers/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,xunzhang/dynet,xunzhang/dynet,clab/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,chunyang-wen/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,xunzhang/dynet,shuheik/dynet