Fix CoAP API function name mismatch (#1317)
Fix CoAP API function name mismatch (#1317)

Refractor in implementation:
* otCoapGetCurrentOption => otCoapHeaderGetCurrentOption
* otCoapGetNextOption => otCoapHeaderGetNextOption
jwhui/openthread,mszczodrak/openthread,bukepo/openthread,lanyuwen/openthread,mszczodrak/openthread,lanyuwen/openthread,jjlee9/openthread,nibanks/openthread,aeliot/openthread,gandreello/openthread,fbsder/openthread,jjlee9/openthread,openthread/openthread,fbsder/openthread,Zolertia/openthread,nibanks/openthread,JakubBrachTieto/openthread,GiedriusM/openthread,xiaom-GitHub/openthread,bukepo/openthread,chshu/openthread,aeliot/openthread,Zolertia/openthread,jwhui/openthread,vaas-krish/openthread,fbsder/openthread,bukepo/openthread,jjlee3/openthread,jjlee3/openthread,vaas-krish/openthread,erja-gp/openthread,aeliot/openthread,pvanhorn/openthread,LeZhang2016/openthread,pvanhorn/openthread,LeZhang2016/openthread,lanyuwen/openthread,georgecpr/openthread,librasungirl/openthread,mszczodrak/openthread,xiaom-GitHub/openthread,GiedriusM/openthread,gandreello/openthread,GiedriusM/openthread,mszczodrak/openthread,georgecpr/openthread,JakubBrachTieto/openthread,georgecpr/openthread,JakubBrachTieto/openthread,pvanhorn/openthread,chshu/openthread,vaas-krish/openthread,turon/openthread,Zolertia/openthread,librasungirl/openthread,georgecpr/openthread,chshu/openthread,librasungirl/openthread,fbsder/openthread,erja-gp/openthread,JakubBrachTieto/openthread,lanyuwen/openthread,nibanks/openthread,erja-gp/openthread,librasungirl/openthread,nibanks/openthread,aeliot/openthread,jjlee3/openthread,mszczodrak/openthread,abtink/openthread,GiedriusM/openthread,gandreello/openthread,jwhui/openthread,gandreello/openthread,turon/openthread,gandreello/openthread,GiedriusM/openthread,chshu/openthread,openthread/openthread,srickardti/openthread,vaas-krish/openthread,Zolertia/openthread,erja-gp/openthread,aeliot/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,georgecpr/openthread,lanyuwen/openthread,LeZhang2016/openthread,mszczodrak/openthread,chshu/openthread,turon/openthread,lanyuwen/openthread,vaas-krish/openthread,vaas-krish/openthread,srickardti/openthread,jjlee9/openthread,abtink/openthread,pvanhorn/openthread,jjlee9/openthread,jjlee3/openthread,LeZhang2016/openthread,jwhui/openthread,aeliot/openthread,abtink/openthread,turon/openthread,bukepo/openthread,fbsder/openthread,xiaom-GitHub/openthread,LeZhang2016/openthread,jjlee9/openthread,georgecpr/openthread,erja-gp/openthread,JakubBrachTieto/openthread,nibanks/openthread,abtink/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,pvanhorn/openthread,fbsder/openthread,openthread/openthread,nibanks/openthread,jjlee3/openthread,gandreello/openthread,Zolertia/openthread,erja-gp/openthread,srickardti/openthread,pvanhorn/openthread,chshu/openthread,JakubBrachTieto/openthread,openthread/openthread,jjlee9/openthread,srickardti/openthread