Fix the constructor of `DiscreteNonParametric` #1419

Job	Run time
Julia 1.3 - ubuntu-latest - pull_request	23m 49s
Julia 1.3 - macos-latest - pull_request	25m 58s
Julia 1.3 - windows-latest - pull_request	32m 53s
Documentation	2m 2s
Julia 1 - ubuntu-latest - pull_request	20m 10s
Julia 1 - macos-latest - pull_request	11m 55s
Julia 1 - windows-latest - pull_request	29m 19s
Julia pre - ubuntu-latest - pull_request	27m 28s
Julia pre - macos-latest - pull_request	20m 16s
Julia pre - windows-latest - pull_request	37m 22s
	3h 51m 12s

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