fix(rxjs-adapter): fixes isValidStream, broken by xstream v6.3 
fix(rxjs-adapter): fixes isValidStream, broken by xstream v6.3 

PR #438
usm4n/cyclejs,maskinoshita/cyclejs,maskinoshita/cyclejs,staltz/cycle,staltz/cycle,feliciousx-open-source/cyclejs,cyclejs/cycle-core,cyclejs/cyclejs,usm4n/cyclejs,cyclejs/cyclejs,ntilwalli/cyclejs,usm4n/cyclejs,ntilwalli/cyclejs,ntilwalli/cyclejs,feliciousx-open-source/cyclejs,feliciousx-open-source/cyclejs,feliciousx-open-source/cyclejs,usm4n/cyclejs,maskinoshita/cyclejs,ntilwalli/cyclejs,cyclejs/cycle-core,maskinoshita/cyclejs,cyclejs/cyclejs,cyclejs/cyclejs