Extend peerDependencies to Angular v7 - 6.1.0-beta.1

fix: missing @optional() in EntityCacheDataService ctor param

feat: allow server to respond 204-No Content after saveEntities (john…

…papa#192)

feat: 6.1.0-alpha.2 enhancements (johnpapa#189)

feat: extend sample app with saveEntities example

[WIP] fix: testability improvements for beta-10 (johnpapa#172)

feat: ClearCollections, LoadCollections, testability improvements (beta-10)

Fixes: beta.9 (johnpapa#167)

closes johnpapa#163, 165
* Critical: data service update should send entity data, not Update<T> #165
* Allow Angular peer dependencies to update within v.6
See ChangeLog

refactor: minor EntityActionFactory change - v6.0.2-beta.8 (johnpapa#164

)