switch to isinstance for df_parser type checking
switch to isinstance for df_parser type checking

Signed-off-by: Adam Miller <9d2b519502b7f059da0978e5ef96c2f4172e413b@fedoraproject.org>

projectatomic/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,maxamillion/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,vrutkovs/atomic-reactor,DBuildService/atomic-reactor,jarodwilson/atomic-reactor,jarodwilson/atomic-reactor